Heat the oven to 180°C (160 C in a fan oven), gas 5.
For the cake mixture
Place all the cake mixture ingredients into a food processor and chop up to resemble breadcrumbs.
Tip the mixture out onto the work surface and knead into a smooth dough.
Roll out 3/4 of the mixture approx. 3-4 mm thick and place it in the cake tin.
Press down well around the edges.
Put the rest of the dough back in the food processor.
To make the filling
Mix together the cornflour and cocoa powder.
Mix together the quark, yogurt, baking powder and the sugar and add the cornflour-cocoa mixture.
Beat in the eggs and spread the mixture on top of the dough in the cake tin.
Add the sugar and the flour to the remaining dough in the food processor and mix into crumbs.
Sprinkle the crumble over the topping and bake for 75 minutes in the lower part of the oven.
Halfway through the baking time cover with aluminium foil so that the cake does not get too dark.
Remove the cake from the oven and leave to cool on a wire rack.
When cool, dust with icing sugar.
Stick test: Push a clean, dry wooden stick at a slight angle into the middle of the cake and then remove it. If there is no mixture stuck to the stick and the stick feels dry, then the cake if done.
If not then bake for another 5 minutes and repeat the test.