Caramelized edges, molten, gooey middle. Crispy, crinkly top. Oozy, soft dark chocolate. Sugary golden brown butter poured and molded into bar form. Slightly adapted from Marcel Desaulniers' Double Chunk Butterscotch Bars in his book Death by Chocolate Cookies - I decreased the sugar somewhat, and swapped in dark chocolate for his suggested white and semisweet.
Grease a 9 x 13 pan. Cut a piece of parchment paper to cover the base, including enough extra to overlap two opposing sides and form handles with which you can lift the cooled bars out.
In a medium bowl, whisk the flour, baking powder and salt together. Set aside.
In a large bowl, whisk the melted butter and brown sugar together until blended.
Add the eggs one at a time, mixing well between additions. Add the vanilla and stir to blend.
Add the flour mixture and stir with a spoon or spatula until just blended. Fold in the chocolate chunks.
Pour the batter into the prepared pan and bake on the middle rack of the oven for 25 minutes, or until set in the middle. My preference leans towards under-baked, so I tend to start checking on these 5 minutes early.
Let the bars cool in the pan (if you have the will) and then transfer to a cutting board to cut into squares. If you want them neat and tidy, wipe the knife clean between slices.