For the sponge cake, line the bottom of a springform pan (26 cm Ø) with baking paper. Separate the eggs. Beat the egg white with the whisk of the mixer until stiff, adding 1 pinch of salt and 100 g sugar. Stir in the egg yolk briefly. Mix flour and baking powder and fold in. Spread the mixture into the form. Bake in the preheated oven (electric cooker: 175 °C/circulating air: 150 °C/gas: see manufacturer) for approx. 30 minutes. Let it cool down.
For the buttercream boil up 300 ml milk. Stir the rest of the milk and pudding powder until smooth, pour into the boiling milk, bring to the boil and simmer for about 1 minute while stirring. Pluck the marzipan into small pieces and stir into the hot pudding with the whisk of the mixer. Cover directly with foil in a bowl and let it cool down lukewarm at room temperature.
Wash, clean and roughly dice the strawberries, except for about 6 pieces. Mix with jam. Dice butter. Cream the pudding. Gradually add the butter cubes and whip vigorously.
Cut the cake in half horizontally. Spread the strawberry mixture on the bottom cake layer. Approx. 1⁄4 Add butter cream and smooth it down. Place the upper cake layer on top. Spread the rest of the buttercream on the cake. Make butterflies out of the remaining strawberries and spread the cake on top.
For each butterfly, cut 3 thin slices from a strawberry halved lengthwise. Place 1 slice vertically as body in the cream, 2 slices as wings next to it.