Wash the lemon thoroughly and finely grate the peel. Melt the fat in a small pot. Put the sugar, salt, vanilla sugar, lemon zest and fat into a mixing bowl and whisk with the whisk of the hand mixer until foamy.
Stir in flour and baking powder. Stir in the eggs one by one. Line the bottom of a springform pan (26 cm Ø) with baking paper. Fill in the dough and smooth it down. Bake in the preheated oven (electric: 175 °C/ gas: level 2) for 25-30 minutes.
In the meantime squeeze the lemon. Mix lemon juice and icing sugar. Take the cake out of the oven and remove from the edge of the springform pan with a sharp knife. Prick the cake several times with a wooden skewer and drizzle the lemon icing over it.
Let the cake cool down on a grid. Then remove from the tin and remove the baking paper. Spread the lemon curd on the cake. Serve the cake decorated with lime and lemon slices as desired.
Results in about 12 pieces.