You will need a 9'/23 cm baking pan. Lightly grease the pan, dust with flour (or line with parchment) and set aside.Start by preparing the fruit filling mixture. In a pan combine the blackberries with the prunes and 2 tablespoons of sugar and cook over a medium heat squashing the fruit with a fork until the mixture thickens. This will take about 6 minutes. Set aside to cool while you prepare the pastry.
To make the dough to the food processor add the flour, baking powder, salt, 120g of sugar, egg yolks, butter, vanilla extract and yogurt and pulse until the dough comes together.
Cut the pastry dough in half then cut one half into 2 equal parts. Place one of these small parts on top of a flat surface, add the cocoa powder and knead it in. Place all the pastry parts in the freeze for 15 minutes (it will be easier to grate them later). Preheat the oven to 375 F/ 180 C/ gas mark 4.
Remove the larger pastry dough from the freezer, cut into thin slices (approx. 5mm thick), arrange over the bottom of the baking pan and press down spreading as evenly as you can (use your knuckles). Cover with the fruit mixture (once cooled) and even out using a large spoon.
Using a box grater coarsely grate the cocoa dough over the fruit (without pressing down!).
Beat the egg whites with a pinch of salt until soft peaks form, then add 1 tablespoon of sugar beating all the time until the mixture is thick and firm. Spread the mixture over the cocoa pastry layer (without pressing down!).
Coarsely grate the remaining small dough over the whole thing and bake in the centre of the oven for 50 minutes. If the top starts to brown too much cover loosely with parchment after about 40 minutes.
Remove from the oven and set aside to cool completely before cutting. Dust with powdered sugar before serving.
Use fresh or frozen blackberries.
Do NOT press the crumb layers down.
Allow the cake to cool completely before cutting it. Serve as is or with a dusting of powdered sugar.