Mix quark, mascarpone, 4 tablespoons lemon juice and sugar. Chop 20 chocolate mint bars and stir them in. Peel 5 bananas, cut in half lengthwise and sprinkle with 3 tablespoons of lemon juice. Dab the banana halves dry and cover the sponge cake base with them. Spread the quark mixture on top and chill the cake for about 2 hours. Peel 1 banana, cut diagonally into thicker slices and sprinkle with 1 tablespoon of lemon juice.
Cut the rest of the mint bar diagonally in half. Decorate the cake with banana slices, mint bars and mint leaves. Warm the jam and pass through a sieve. Coarsely brush the edge of the fruit base with the jam. Sprinkle with chocolate shavings
2 1/2 hours waiting time