Beat the eggs, sugar and 2 tablespoons of hot water with the whisk of the hand mixer until thick and creamy. Mix flour, starch and baking powder, sieve over the mixture and carefully fold in.
Pour into a springform pan (24 cm Ø) lined at the bottom with baking paper. Bake in a preheated oven (electric: 200 °C/ gas: level 3) for 17-20 minutes. Allow to cool and cut through horizontally twice.
Spread the lower base with jam, place the second base on top. For the filling, put cream powder in a mixing bowl. Add milk and 250 g cream. Using the whisks of the hand mixer, first beat for 1 minute on lowest setting, then for 3 minutes on highest setting.
Spread half of the mousse on the second sponge cake base, place the remaining base on top. Spread the rest of the cream all around the cake. Chill for about 3 hours. Whip the rest of the cream until stiff for decoration.
Pour into a piping bag with a small perforated nozzle. Pour crosswise over the cake. Dust with cocoa and decorate with chocolate truffles. Makes about 12 pieces.