Lemon Rhubarb Bars

These lemon rhubarb bars feature a sweet and tart lemon custard that envelops a generous amount of vibrantly hued rhubarb. With a buttery shortbread base and a decorative toasted meringue topping, they're are an excellent way to celebrate the arrival of rhubarb in gardens and local shops.

I've been eagerly awaiting the arrival of rhubarb to my local grocery stores. Just as citrus fruits signal the shift of seasons into winter, rhubarb is firmly stuck in my mind as one of the first signs that spring is truly on its way.

Around the same time that the flowering fruit trees burst into colorful cotton candy blooms, those first brightly pink and red hued stalks appear. The growing season for rhubarb is brief, so I try to take advantage whenever I see it. I've used it in cakes, tarts and pies - including this rhubarb frangipane tart and a strawberry lime rhubarb slab pie.

Here I wanted to incorporate the rhubarb into a recipe for shortbread bars, and took inspiration from an old fashioned dessert favorite, rhubarb custard dessert bars, but with a lemony custard filling and no mysterious "whipped toppings."

Not quite a lemon bar, nor a rhubarb custard bar, I've married the two by adding three types of lemon (freshly squeezed lemon juice, lemon zest and lemon oil) and a generous amount of diced rhubarb.

The origin matters less than the end result: a vibrant, brightly flavored bar packed with a balance of sweet custard, rich shortbread crust, and the inevitable zing from tart lemon and rhubarb combined. It's all tempered with a sweet decorative layer of toasted Swiss meringue that ties it all together nicely.

Ingredients

* See recipe card for detailed list of ingredients and quantities.

Unsalted butter - Using unsalted butter allows you to control the total amount of salt in the recipe.

Lemon oil - I use lemon oil in this recipe to add an extra boost of lemon flavor to the custard filling. Lemon oil is more concentrated and flavorful than lemon extract.

The swiss meringue topping

To top these bars, I use a Swiss meringue. It requires a bit more effort than a common meringue, but the result is a stronger, more stable structure that holds up beautifully over time. While these bars are best enjoyed fresh, I found that leftover bars with meringue topping held up in the fridge for several days with no weeping at all.

Easy spreading or decorative piping?

This recipe makes enough meringue to fully cover the top of the bars. However, you can choose how you'd like to apply it, as each method has its own advantage:

• Spread a Full Layer: This is the easiest method. You'll cover the vibrant pink and red hues of the rhubarb, but you'll get a perfect bite of sweet meringue with every bit of lemon rhubarb bar.
• Pipe Decorative Swirls: Pipe individual meringue swirls or kisses onto each bar. This preserves the beautiful color of the rhubarb filling and creates an elegant, bakery-style look. If you choose this option, you will have some leftover meringue.

How to use leftover meringue

Don't let any of that delicious meringue go to waste! It's perfect for making light, crunchy meringue kisses.

1. Pipe small swirls or "kisses" onto a baking sheet lined with parchment paper, leaving about an inch of space between each one.
2. Bake in a 200°F (93°C) oven for 60-90 minutes, or until the meringues are dry to the touch and can be easily lifted off the parchment paper with their bases intact.
3. Turn the oven off and leave the meringues inside to cool completely; this helps prevent them from cracking.

Equipment

8-inch square metal baking pan: I use and recommend this USA Bakeware square pan. Its sharp 90-degree corners create beautifully clean-looking bars, and the quality ensures they bake evenly every time.

16 inch piping bag - A large piping bag is great for adding decorative details with the Swiss meringue.

Piping Tip: Use a St. Honoré tip or get creative with any large tip that has a similar opening.

If you choose to pipe decorative meringue patterns rather than covering the whole top, you'll likely end up with some leftover meringue.

To avoid food waste, consider making piped meringue kisses. Pipe onto parchment and cook in a 200 degree F (93 degrees C) oven for 2 to 2 ½ hours.

Frequently asked questions

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Lemon Rhubarb Bars

Refreshingly tart rhubarb enveloped in a sweet lemony custard layer with a buttery shortbread base. Topped with toasted Swiss meringue, these bars are an excellent way to highlight the vibrant pink color and unique flavor of rhubarb as it makes its all-too-brief appearance in spring.

AuthorKathleen Culver

Prep time20 minutes minutes

Cook time45 minutes minutes

Chill time1 hour hour 30 minutes minutes

Total time2 hours hours 35 minutes minutes

Servings16 bars (8 inch square pan)

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Cook ModePrevent your screen from going dark

INGREDIENTS

Metric - US

SHORTBREAD CRUST

• ▢ 113 grams unsalted butter, chilled, cubed
• ▢ 50 grams granulated sugar
• ▢ 120 grams all purpose flour
• ▢ ¼ teaspoon kosher salt, Diamond Crystal

LEMON RHUBARB CUSTARD LAYER

• ▢ 23 grams all purpose flour
• ▢ 150 grams (¾ cup) granulated sugar
• ▢ 2 large eggs, beaten
• ▢ 60 mL heavy cream
• ▢ lemon zest, from 1 large lemon
• ▢ 60 mL lemon juice, freshly squeezed, from the same lemon
• ▢ 2.5 mL lemon oil
• ▢ 366 grams rhubarb, diced

SWISS MERINGUE TOPPING

• ▢ 100 grams egg whites, ~3 large egg whites
• ▢ 150 grams granulated sugar
• ▢ 1.25 mL vanilla extract

INSTRUCTIONS

SHORTBREAD LAYER

• Preheat the oven to 350° F (177 C, gas mark 4).
• Line an 8" (20 cm) square pan with parchment paper, extending a flap of parchment over opposite sides as a handle to aid in removing the bars later.
• In the bowl of a stand mixer fitted with the paddle attachment, combine the butter, sugar, flour, and salt and mix until the dough begins to come together with a lumpy, sandy texture.
  113 grams unsalted butter, 50 grams granulated sugar, 120 grams all purpose flour, ¼ teaspoon kosher salt
• Dump the dough into the prepared pan and press with your fingers until it is spread evenly into all areas of the pan. Cover with plastic wrap and use the flat base of a drinking glass or the bottom of a measuring cup to smooth the top and firmly pack down the dough. Chill in the fridge for 30 minutes.
• Bake the shortbread for 10-12 minutes, until top has gone from shiny to dull, shortbread is just baked through, barely starting to turn golden brown, and beginning to smell like toasted butter. Remove from oven and set aside to cool on a baking rack.

LEMON RHUBARB CUSTARD LAYER

• In a large mixing bowl, whisk together flour and sugar.
  23 grams all purpose flour, 150 grams (¾ cup) granulated sugar
• Add eggs and whisk to combine.
  2 large eggs
• Add heavy cream. Zest the lemon directly into the cream. Add lemon juice and lemon oil and whisk until completely blended.
  60 mL heavy cream, lemon zest, 60 mL lemon juice, 2.5 mL lemon oil
• With a spatula, fold in diced rhubarb and pour mixture onto cooled shortbread crust. Rap pan firmly on countertop several times to remove air bubbles that may have formed in custard.
  366 grams rhubarb
• Bake at 350° F (177C, gas mark 4) for 40-45 minutes, until custard layer is completely set - it should not jiggle. Place on a metal rack to cool. Once cool, place in refrigerator to set for at least one hour.

SWISS MERINGUE TOPPING

• In a small (1 qt ) saucepan, whisk together egg whites, sugar and vanilla extract. Warm over low heat, whisking frequently, until mixture is warm and granulated sugar has dissolved.
  100 grams egg whites, 150 grams granulated sugar, 1.25 mL vanilla extract
• Rub some of the egg white mixture between your fingers (carefully - it might be hot). If you feel sugar granules, keep mixing until it feels smooth. If you have a thermometer, it should read 160° F (177 C).
• Transfer warm egg mixture to the bowl of a stand mixer fitted with the whisk attachment. Beat until stiff peaks forms and mixture has a glossy sheen - about 5 minutes.

  The meringue should be very thick and hold its shape (no drooping) when the whisk is dipped into the mixture and turned upright.
• Remove fully chilled bars from pan. Place bars onto a metal baking sheet, removing the parchment paper from underneath.

  Kitchen torches and paper do not mix!
• Option 1 - Spread meringue: Scoop the meringue out onto the fully cooled bars. Spread evenly to cover the filling, making swirls with an offset spatula or spoon. Toast the meringue using a torch. Slice into 16 squares, wiping the knife clean in between each cut.
• Option 2 - Pipe meringue: Slice into 16 squares, wiping the knife clean in between each cut. Pipe the meringue onto individual bars using a 16" (40cm) piping bag fitted with a decorative tip. Toast the meringue using a torch.
• Serve promptly. Keep additional bars refrigerated in a single layer in an airtight container.

EQUIPMENT

• 8 inch square baking pan
• 16 inch piping bag
• St. Honore piping tip