Line the bottom of a springform pan (26 cm Ø) with baking paper. Separate the eggs. Beat the egg white until stiff, adding 1 pinch of salt and sugar. Stir in the egg yolk briefly. Sift flour and baking powder over it and fold in carefully. Spread the mixture into the tin. Bake in a hot oven (electric cooker: 175 °C/circulating air: 150 °C/gas: see manufacturer) for approx. 25 minutes. Remove the sponge cake from the edge of the mould and let it cool down.
Mix the cream stiffener with 2 sachets of vanilla sugar. Whip 700 g cream stiff, letting the sugar mixture trickle in. Stir the cranberries until smooth and fold into the cream, together with the lemon zest, up to 3 tbsp. Cut the sponge cake in half horizontally. Place a cake ring around the lower cake base. Spread approx. 2⁄3 of the cream evenly on the cake base. Place the upper cake layer on top and press down gently. Remove the cake from the ring and spread the edge with the remaining cranberry cream. Chill for about 2 hours.
Whip 300 g cream until stiff and allow the remaining vanilla sugar to trickle in. Pour into a piping bag with star-shaped spout and squirt onto the cake. Decorate with 3 tablespoons cranberries.
Spray cream in the form of cake pieces on the top cake base: To do this, move the piping bag in a zigzag motion from the outside inwards. Looks nice and makes it easier to cut.