Drain mandarin oranges on a sieve, collecting the juice. Cut the breakfast cake into 8 slices. Line a mould (approx. 24 x 14 cm) with them. Mix coffee and liqueur and soak the cake in it. Spread mandarin oranges on the cake. Mix mascarpone, sour cream, 6 tablespoons mandarin juice and vanilla sugar.
Spread the cream on the mandarins and press dents into it with a spoon. Chill the Tiramisu for about 2 hours. Dust with cocoa. Cut into 8 pieces and arrange on plates. Decorate with tangerine slices and cocoa if desired.
2 hours waiting time