Separate the eggs, and mix the yolks with the sugar by a hand mixer till it will be pale in color, then mix it with the water.

Whip the egg whites till you get a stable foam.

Incorporate the flour (or the flour plus cocoa) and the baking powder into the egg yolk mixture.

Fold the egg white foam gently into the mixture.

Pour the batter into a lined medium size (diameter: 18-20 cm) cake tin.

Bake the sponge on 180 °C till it will pass the toothpick test.

Let the crust to chill till room temperature before the filling.

Parisian cream filling

Boil the cream and the sugar on high temperature for 7 minutes.

Add the butter and boil it on high temperature for 5 minutes.

Take off the pot from the stove and incorporate the cocoa powder into the hot, but not boiling mixture till you get a smooth, silky texture.

When its on room temperature, put it in the fridge till it will be solid like butter. It takes for a while, so dont forget it when you schedule the preparation.

When the filling has a firm, butter texture whip it till you get a spreadable, smooth chocolate cream.

Now your cream filling is ready to fill in between the crust layers and/or cover the cake with it.