Layer: Separate the egg whites from the yolks. Heat the yolks with 2/3 of the sugar and beat them until whitened. Remove from the heat, add the lemon zest to them.
Beat the whites to snow with the remaining sugar. Add the yolks, cocoa, flour and walnuts. Stir everything very well and pour it onto baking paper in a round tray. Bake in a moderate oven to a dry toothpick.
Cream: Melt the butter slightly, add the sugar, walnuts, milk, vanilla and cocoa. Stir everything until whitened.
Syrup: Boil the water and sugar for 5 min., then add the rum.
Cut the layer horizontally into 2 parts, syrup them, put cream in between them. Smear the top and sides with cream as well, then sprinkle on ground walnuts.