Countertop sheets: I used a 28/38 cm tray that I lined with baking paper. I mixed the egg whites with a powder of salt well, then I added the sugar and I continued to mix until I obtained a strong meringue. I added walnuts, flour, cocoa and baking powder and mixed, taking care to mix with slow movements from bottom to top. I spread the obtained composition in the prepared tray, I leveled it well and I put the tray in the preheated oven, it is ready when it passes the toothpick test. Remove the tray from the oven, let the countertop sheet cool and do the same with the second countertop sheet. As long as the two sheets cool, we start the cream.
Cream: In a saucepan we put the yolks with the sugar, we place the pan in another pan in which the water boils, that is to put it on the steam bath. Mix well until the sugar melts, then add the finely chopped chocolate, mix and mix until the chocolate melts, then add the cocoa. Mix well until it starts to thicken, remove from the heat and let it cool. Butter the butter at room temperature until it becomes a foam, then take it with a spoonful of the cream that has cooled and mix it lightly, proceed as soon as you finish the chocolate cream to incorporate it in the butter.
Assembly: We place the first countertop sheet on a tray, spread the cream evenly, put the second sheet and press lightly to stick the cream on both sheets. Then we start to deal with the icing. Chocolate together with sour cream on the fire, mix until the chocolate melts, taking care not to reach the boiling point. Spread the icing over the cake while it is hot, then sprinkle the candies. Leave it in the fridge until the next day, when we portion it.