• I recommend using a kitchen scale to make this cake, cup measuring is too imprecise and might cause problems.
  
  Start with the topping and the pudding for the buttercream, they should have enough time to cool.

Buttercream:

• For the pudding mix the flour and the cornstarch with a small amount of the measured milk, enough to obtain a thick yet pourable paste. Whisk well to avoid clumps.
• Heat the rest of the milk until it comes to a boil. Add the flour-cornstarch mixture while whisking all the time and keep whisking vigorously until the mixture thickens, it will only take moments.
• Remove from the heat, stir in vanilla extract to taste (the pudding is not sweetened yet, so don't get a shock, the sugar comes in with the butter), and place a piece of cling film directly on top of the pudding.
• Let get cool at room temperature, it will take several hours, so I prefer to make the pudding and the ganache one day in advance.

Ganache:

• Chop the chocolate very finely or process it in the food processor, place it in a bowl.
• Heat the cream in a small pot.
• Pour the hot cream over the chocolate and let stand for 5 minutes. Stir well until the chocolate is melted and shiny. Cover with cling film, let cool, and place in the fridge for several hours or overnight.

Cocoa dough layers:

• Preheat the oven to 180 degrees Celsius/ 350 degrees Fahrenheit.
• Choose a baking dish of about 23x33 cm/ 9x13 inches and cut 3 sheets of parchment paper. You will only need the baking dish for baking the sponge, but the cocoa layers have to be rolled the same size as this pan.
• Place the icing sugar, soft butter, milk, honey, eggs, vanilla extract, cocoa, flour, and baking powder in the food processor and process shortly until well mixed.
• Place one sheet of parchment paper on the working surface and place half of the cocoa dough on it. Press the dough to a rough rectangle with your hands, cover it with another sheet of parchment paper and roll it to a rectangle of the same size as the tin you will be using to bake the sponge.
• Remove the upper parchment paper and transfer the dough together with its lower parchment paper sheet onto a baking tray. Bake for about 10-12 minutes.
• In the meantime roll the second half of the cocoa dough just as explained before and bake as well. Let cool.

Vanilla sponge:

• While the second cocoa layer bakes, prepare the sponge mixture.
• Lightly butter the rectangular baking dish you chose before and line it with parchment paper.
• Separate the eggs. Beat the egg whites until stiff, slowly add the sugar and continue beating until the egg whites are stiff and glossy. In another bowl mix the egg yolks and the warm water. Carefully incorporate into the egg whites.
• In another bowl mix together the flour, baking powder, and salt. Sift over the egg mixture and incorporate it carefully with a plastic spatula.
• Pour the mixture into the prepared pan and bake in the preheated oven for about 25-30 minutes or until a toothpick inserted in the middle of the cake comes out clean.

Vanilla buttercream:

• Read the tips above before making the buttercream.
• To make the buttercream start by beating the soft butter until white and fluffy, about 3-4 minutes.
• Add the icing sugar and continue beating until fluffy.
• Slowly start adding the pudding, one tablespoon at a time while beating all the time. Only add the next tablespoon of pudding to the buttercream when the previous one has been fully incorporated.

Topping:

• Remove the ganache from the refrigerator about 30-60 minutes in advance. Beat the ganache until fluffy, taste, and add a bit of icing sugar if you feel it necessary (I didn't).

Assemble the cake:

• Place one cocoa layer on a suitable rectangular serving dish.
• Spread half of the vanilla buttercream on top and level nicely.
• Place the sponge on top and cover it with the remaining vanilla buttercream.
• Place the second cocoa layer on top and press lightly.
• Spread the ganache on top and sprinkle the cake with the chocolate flakes.
• Cover carefully with some aluminum foil without ruining the topping and place in the fridge for at least 4-5 hours or better overnight.
• Cut into small squares and serve.