Preheat oven to 180C. For the chocolate cookie crumb combine all ingredients in a bowl and use your hands to thoroughly combine. The mixture should form clusters quite quickly. Spread the clusters on a tray lined with baking paper and bake for 20 minutes. Set aside to harden and cool. 

Grease and line a 20cm x 30cm high-sided (15cm) baking tray with plastic wrap, ensuring there is plenty of overhang. Combine the chocolate ice-cream and dark chocolate chunks in a bowl until combined. Pour into the lined tray and place in the freezer to harden – at least a few hours.

Repeat with the raspberry gelato, spreading it over the chocolate layer. Return to the freezer to set.

Preheat the oven grill to high. Place the marshmallows onto a lightly greased sheet of baking paper. Place in the oven to toast for 1 to 2 minutes, watching regularly as they can turn from caramel to burnt in the blink of an eye. Remove and set aside.

Remove the ice-cream tray from the freezer and cover liberally with the cookie crumb.  Carefully remove the toasted marshmallows from the sheet of baking paper using a spatula, and place on top of the cookie crumb (don't worry, they hold their shape very well during transit). Use a hot wet knife to cut into the ice-cream cake and serve immediately.

Note: This makes more cookie crumb than required for this recipe. Any leftovers will store covered in the fridge for one month.