Oil the bottom of a square springform pan (24 x 24 cm; alternatively round, 26 cm Ø). Melt butter in a pot, remove from heat and let it cool down a little. Fill the biscuits into a large freezer bag, close it and roll a cake roll over them until the biscuits are finely crumbled.
Mix with the liquid butter. Press into the mould to form a base. Chill for about 10 minutes.
In the meantime, mix mascarpone, curd and sugar with the whisks of the mixer. Spread the mixture loosely on the biscuit base and cover and chill the cake for at least 2 hours.
Remove the cake from the tin and cut it into strips (approx. 2 x 8 cm each). Decorate with plums as desired.