Beat the egg yolks with the sugar until whitened and the potato flour a spoon at a time, while stirring constantly. Add the separately beaten to snow egg whites, mixed with a pinch of salt, stir carefully and add the cocoa. It needs to be a smooth and porous mixture.
Bake the mixture into 2 identical cake layers in identical forms, covered with baking paper, or greased. For the cream, beat the butter with the sugar into foam, add the melted chocolate (not hot), rum and the egg yolks one at a time and the vanilla. All products need be at roughly the same temperature.
Finally mix the cream with the egg whites, beaten separately to snow. Optionally, you can cut one of the cake layers into 2 separate ones. Once cooled, stick the cake layers together with cream and then, brush the entire cake with cream. Top it with grated chocolate and sifted cocoa.