One day in advance, make the pistachio cream and pipe it into pyramid molds (1 by 1 in/3 by 3 cm each). Freeze the molds overnight.
Using a pastry brush, apply a thin layer of pâte à glacer on top of the sponge and let it set. Once set, flip the sponge over and soak it with simple syrup and kirsch.
Slice the cake into squares slightly larger than 3½ b