Line a slice tin measuring approximately 17cm x 26cm x 4cm with baking paper.
Crush biscuits in a food processor down to a fine crumb. Add melted butter and stir to combine - mixture should resemble sticky breadcrumbs. Press into prepared tin (the bottom of a glass works a treat). Place in fridge to set.
Using a food processor or electric mixer whiz the cream and condensed milk together until they just thicken (you don't want to curdle your cream), stir through lamington cubes and pour over prepared biscuit base.
Cover with foil and freeze for 6 hours or overnight.
Cut slice into squares, drizzle with chocolate sauce and serve immediately.
Keep any leftovers in freezer for up to 2 weeks if they last that long!