Crush the biscuits to fine crumbs in a food processor. Put the crumbs in a large mixing bowl and add the melted butter, stir thoroughly.Tip them into your baking tin, pressing down firmly with a spoon. Place in the fridge until set.
Layer 1: Spoon the chocolate gelato over the crumb base, pressing down evenly.Place in the freezer until set.
Layer 2: Repeat last two steps with the raspberry sorbet.
Layer 3: Repeat again with the vanilla ice cream.
Cover with baking parchment and leave in the freezer overnight to firm up.
When you are ready to serve the gelato cake, remove the parchment, place a serving plate on top, invert the pan and then invert again on to another plate so that it sits base-down. Serve in slices.