Beat 4 egg yolks with the sugar and water into a cream that sticks to the whisks and does not fall off from them. Add the flour that you've sifted with the baking powder and then - the beaten to snow egg whites. Mix with the walnuts. Pour into a greased and lightly floured tray and bake the layer in a moderate oven.
Boil the syrup from the water and sugar listed for it. Cut the cooled layer into 2 and pour on the sugar syrup and the cognac.
For the white cream, boil the milk with the sugar and while constantly stirring, add in the flour that's been diluted in a little cold milk. After thickening, remove from the stove and let it cool. In the meantime, beat the soft butter and sugar into fluffy cream. Add in the cooled cream and stir.
Smear the top half of the cake layer with this cream. Put the bottom half on top and smear the entire cake with the yellow cream.
On top, you can make a decoration of your choice.