Separate eggs. Beat the egg whites until stiff, adding salt and sugar. Fold in the egg yolks briefly. Mix flour and baking powder, sieve over the egg mixture. Add lemon zest and fold in carefully.
Line the bottom of a springform pan (26 cm Ø) with baking paper. Fill in sponge cake mixture and smooth it down. Bake in the preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: see manufacturer) for approx. 25 minutes.
Take the cake out of the oven, remove from the edge and let it cool down.
Mix the cream firmer with 2 sachets of vanillin sugar. Whip 700 g of cream until stiff, allow the firming agent mixture to trickle in. Fold in 275 g cranberries. Cut the sponge cake base in half horizontally. Place a cake ring around the lower cake base.
Spread about 2/3 of the cream on the base and smooth it down. Place the upper cake base on top. Remove the cake from the ring and spread the edge with the remaining cranberry cream. Chill for about 2 hours.
Whip 500 g cream until stiff, pour in 2 packets of vanilla sugar. Pour the cream into a piping bag with a large star-shaped spout. Spray the cream onto the cake from the outside to the inside. Spread 3 tbsp. cranberries in patches on top.