125grams (½ cup chopped and packed, a bit over ¼ pound)Italian plums, pitted
300grams (2½ cups)powdered sugar, sifted
STREUSEL TOPPING (AND FILLING)
How to make streusel: in a medium bowl, stir together all streusel ingredients. There should be no dry flour remaining. Set aside to cool and firm up slightly while making the cake.
PLUM COFFEE CAKE
Preheat oven to 350° Fahrenheit (177° Celsius, Gas mark 4) with a rack in the center of the oven.
Prepare a 9" (23cm) springform pan by placing a circle of parchment paper in the base and greasing the sides.
In a medium bowl, whisk together the flour, cardamom, cinnamon and black pepper. Set aside.
In a small bowl, whisk together the sour cream and the milk. Set aside.
In the bowl of a stand mixer fitted with the paddle attachment, cream the butter and sugar together until light and fluffy, approximately 4-5 minutes. Scrape down the bowl. Add salt, baking soda and baking powder and mix until combined. Add eggs one at a time, mixing well to incorporate each. Mix in vanilla extract.
Scrape the sides of the bowl well and add half of the flour mixture. Mix at low speed until blended. Add sour cream mixture, and mix to combine. Add remaining flour and mix just until blended. Scrape the bowl with a spatula to ensure all flour has been incorporated.
Pour half of the batter into the prepared springform pan and smooth to the edges with a mini offset spatula or the back of a spoon. Distribute half of the prepared streusel evenly on top of the batter, breaking it up with your fingers into a mixture of pea, blueberry, and sandy crumb sized amounts. Add the remaining batter and spread carefully over the streusel to cover it completely. Ensure the batter extends to the edges of the pan.
Distribute the diced plums in an even layer on top of the cake batter. Add remainder of streusel to the top of the cake. Bake for 50-55 minutes, until golden brown, a toothpick inserted into the center comes out clean, and the top springs back lightly when touched.
Cool on a baking rack. Once cooled, run a knife around the border of the springform pan and then release the pan. Place onto a serving platter.
Place all plum glaze ingredients into the bowl of a blender and process on low until incorporated. Scrape down the sides of the blender and process on high until pureed. For a smooth glaze, pass through a strainer to remove leftover plum skins.
Drizzle glaze onto cooled plum coffee cake before slicing (it will harden to a firm glaze), or serve in a small pitcher alongside the cake to be poured onto individual slices. If the glaze is made in advance, it will crust over. Keep the surface covered with a piece of plastic wrap until serving, and stir before using.