You will need a 9 inch (23 cm) pan. Lightly grease the sides, dust with flour 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.
In a large bowl combine the flour, baking powder and salt and stir with a whisk. Add the egg yolks, vanilla, 120 g of sugar and cubed butter. Stir with a spoon to combine then knead to make a smooth pastry dough (I find it easier to knead on top of a flat surface rather than in a bowl).
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 the 2 small pastry parts on top of a plate and freeze for 15 minutes (it will be easier to grate and turn them into crumbs later). Preheat the oven to 375 F/ 180 C/ gas mark 4.
Press the larger pastry dough into the bottom of the pan spreading as evenly as you can. 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 stiff peaks form, then beat in 1 tablespoon of sugar and spread the mixture over the cocoa crumbs (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 (optional).
Use fresh or frozen blackberries.
Do NOT press the crumb layers down.
Make ahead: You can prepare the cake and refrigerate for several hours before baking it.
Allow the cake to cool completely before cutting it. Serve as is or with a dusting of powdered sugar.