Preheat the oven to 160 °C (convection oven).
Preheat the oven to 160 °C (convection oven).
For the sponge base: separate the eggs. Whisk the egg yolks with half of the sugar, the vanilla sugar, salt, lemon juice, lemon zest and rum flavouring until fluffy.
Whisk the egg whites until fluffy. Add the rest of the sugar and continue to whisk until stiff.
Sift the flour and fold it into the egg yolk mixture alternately with the egg white.
Fill into a greased cake tin and bake in the preheated oven for approx. 35 minutes.
Tip: Refine the cream with finely grated lime zest.