Spread one-sixth (about 100g) of the cooled custard onto the base of a 20cm-wide trifle bowl that’s at least 14cm deep. Arrange eight of the cookies in an even layer on top. Spread one-sixth (about 160g) of the mascarpone mixture on top in an even layer. Repeat until you have six layers of cookies, finishing with a layer of the creamy mascarpone mixture. Chill in the fridge for at least 8 hours or overnight. This can be prepared up to 24 hours in advance.