Cut the pineapple into slices, peel and cut out the woody centre. Cut the pineapple slices into pieces. Heat butter in a pan. Add 50 g sugar and melt in the butter.
Add the pineapple and steam for about 5 minutes, turning occasionally. Leave to cool. Line a glass mould (approx. 16 x 21 cm; 1.5 litres capacity) with sponge cake. Mix pineapple juice and liqueur and drizzle over the sponge cake.
Spread half of the pineapple chunks on top. Beat the egg yolks, 3 tablespoons of hot water and the remaining sugar with the whisks of the hand mixer until frothy. Add lemon zest. Mix mascarpone and yoghurt and stir into the egg mixture by the spoonful.
Wash mint, dab dry, pluck leaves from the stalks and cut into fine strips. Fold into the mascarpone cream. Spread half of the cream on the pineapple pieces and smooth it down. Spread the rest of the pineapple pieces, except 3 for decorating, on top of it and cover with the rest of the mascarpone cream.
Chill for at least 2 hours. Dust with cocoa before serving. Decorate with the remaining pineapple pieces and sprinkle with pistachios. Serve decorated with mint if desired.