Butter a 25 x 20cm tin and line with baking parchment. Blitz the shortbread biscuits in a food processor until they resemble breadcrumbs. Or, put them into a freezer bag and bash with a rolling pin. Melt 75g of the butter and stir in the shortbread crumbs. Press the buttery crumbs into the base of the prepared tin using the back of the spoon, making sure the surface is level. Chill for 30 mins until firm and set.

Remove the ice cream from the freezer 5 mins before using so it softens slightly and can be easily scooped (do not let it melt). Dollop the softened ice cream evenly over the shortbread base and spread it to the corners using a small spatula or knife. For an even, smooth surface, dip the spatula in a mug of boiling water and pat dry before using. Freeze for at least 4 hrs until set.

Spoon the caramel over the set ice cream and spread into the corners, smoothing the surface. Sprinkle over a pinch of sea salt flakes, then return to the freezer while you make the topping.

Melt the dark chocolate and remaining butter in a heatproof bowl set over a pan of simmering water. Or, do this in 20-second bursts in the microwave. Leave to cool for 5-10 mins, or until it is room temperature. Melt the white chocolate in the same way. Pour the dark chocolate mixture over the set caramel and spread it to the corners, then drizzle over the white chocolate in a zig-zag pattern. Return to the freezer for at least 24 hrs. Cut into 12 rectangles and serve immediately – you can run the knife under hot water to make slicing easier.