Bake the sponge cake in a rectangular mold. Once cooled, slice into long strips. Line a 10-inch stainless steel bowl or spherical mold with the sponge strips.
Simmer the water with sugar and rum. Lightly soak the sponge lining using a brush or squeeze bottle.
Prepare two batches of pastry cream: one regular, one chocolate (replace 20g of cornstarch with cocoa powder). Fold each with whipped cream.
Layer chocolate cream first, followed by regular cream, adding chopped dark chocolate throughout. Cover with more sponge strips, soak lightly, and freeze for 2 hours.
Unmold the Zuccotto, spread whipped cream over the top, dust with sponge crumbs, powdered sugar, and cocoa powder. Serve chilled.
