Cut the chocolate in pieces and melt it in bain-marie.
Divide the eggs into yolks and whites.
Beat the yolks separately until creamy and add them to the chocolate, while stirring well.
Beat the egg whites in meringue and add them to the chocolate mixture little by little, while stirring constantly.
Grease a greaseproof paper. Then lay it over a tray, pour over the mixture, smooth it with a spatula and bake in preheated oven for 25 – 30 minutes at 170C.
Remove the sponge cake from the oven. When cool, put it on a cotton towel and let it for at least 8 hours.
Prepare the cream as follows: With a mixer, beat the sugar with the sour cream until it gets into a fluffy whipped cream. Then add the liqueur and the cocoa and keep beating until you get a homogenized mixture.
On clean kitchen countertop open the sponge cake and apply the cream with a spatula, in order to cover all the surface of the cake.
Roll the cake and place it on a platter with the edges of the cake from the underside.
Sprinkle with powdered sugar and the cocoa and put the cake in the fridge. Serve after 4-5 hours.