Mix egg yolks. Gradually add vanilla sugar, sugar, salt, juice of one lemon and lukewarm water. Meanwhile, whip the mixture until it is creamy. Whip egg whites until stiff enough to leave a knife cut in them. Beat in sifted powdered sugar and the lemon zest. Slide this foam onto the egg yolk cream. Sift flour over it.
Fold in with a whisk.
From this dough were baked 8 to 9 narrow cake layers. To do this, grease a cake springform pan of 26 cm ø each, pour in the dough 2 to 3 mm and place in the heated oven.
Bake for 10 to 12 min. per base. Electric oven 200 °C Gas oven level 4 Cool the bases.
For the chocolate buttercream, start by putting the sugar in a saucepan and pour enough hot water to just cover the sugar. Bring to a boil while stirring, then cool. Cream butter with a pinch of salt in a baking bowl, add cocoa and mix gently to avoid dusting. Gradually stir the sugar syrup into the butter.
Finally add rum and vanilla sugar. Stir vigorously. Spread this chocolate buttercream evenly on the cake layers.
Leave one cake layer uncoated. Assemble into a cake. Spread edges of cake evenly with buttercream. Place remaining buttercream in a piping bag and set aside to cool. Hold cake diagonally and dust edge evenly with cocoa. Place cake in refrigerator for one hour. I