Mix the quark, sugar and milk together until smooth. Beat the cream and vanilla sugar until stiff. Squeeze the lemon. Peel and slice the banana and drizzle with the lemon juice.
Stir melted chocolate into the quark. Fold in the cream. Set aside a few slices of banana. Fold the remaining banana slices into the cream. Pour the cream into a bowl and garnish with the retained banana slices.