Grease the fat pan of the oven (approx. 32 x 39 cm) and dust with flour. Beat the eggs, 300 g sugar and vanilla sugar for approx. 5 minutes with the whisk of the hand mixer until thick and creamy.
Mix flour and baking powder, stir in briefly. Also briefly stir in oil and lemonade. Pour the dough onto the fat pan and spread evenly. Bake in a preheated oven (electric cooker: 150 °C/ circulating air: 125 °C/ gas: see
manufacturer) Bake for 25-30 minutes. Remove and let cool on a cake rack.
Stir mascarpone, 250 g sugar, cream firming agent, lemon peel and juice until smooth. Whip cream until stiff and fold into the mascarpone in 2 portions. Spread the cream on the base and smooth it down. Cut the cake into squares (approx. 6 x 6 cm) and chill until serving.
Decorate with lemon wedges and possibly with straws.