To make the layer, beat 4 egg yolks and 1 whole egg with 1/2 the sugar until whitened.
Beat the 4 whites with the other 1/2 of the sugar to snow, add it to the dough. Slowly pour in the baking powder, grated rind of the lemon and flour (as needed to obtain a cake-like batter).
Pour the dough into a rectangular tray and bake in a preheated 338°F (170 °C) oven until ready (don't over-bake it). Put the baked cake layer onto a towel and wrap up into a roll immediately after taking it out of the oven.
Once it cools, unroll it and smear with cream, made by mixing together the eggs, sugar, flour and milk and putting them to boil until thickened, while stirring.
Add the grated rind of a lemon, its juice and butter to the cream. Cool it and smear the roll with this cream, then roll it back up. Optionally, smear with cream on top or sprinkle powdered sugar.