Beat the butter with powdered sugar into fluffy cream. In a water bath on the stove, beat the egg yolks with the granulated sugar until the mixture thickens. Be careful not to boil the eggs and make lumps.
Allow the yolk mixture to cool and spoon by spoon add it to the butter cream, constantly whipping. Finally mix with 1/2 of the coffee. Leave the cream as is, or mix it with the beaten to snow egg whites.
Warm the remaining coffee with the milk. Dip each biscuit in this milky coffee mixture and arrange them in a suitable tray. Pour part of the cream on top and repeat until you've built the entire cake.
Finally, spread it evenly with cream and make a decoration of grated chocolate or wafers.