Preheat the oven to 170C / 150C fan, then grease and line the bottom and sides of the cake tin. Put the biscuits into a food processor and blitz until it resembles fine breadcrumbs, then, with the motor running, pour in the melted butter and blitz until it’s all combined. Reserve 1 tablespoon of the buttery biscuit crumbs, then tip the rest into the lined cake tin and use a glass or the back of a spoon to smooth it over and press it firmly down. Bake for 15 minutes, then leave to cool in the tin.
Once the base is cool, spoon the Biscoff spread on top and smooth it over with the back of a spoon. Put into the freezer to set for 30 minutes.
To make the topping, put the cream cheese and cream into a large mixing bowl and use an electric whisk to beat it until smooth. Sieve in the icing sugar and whisk again until it is thick. Spread it over the chilled biscuit filling and sprinkle the reserved biscuit crumbs over it. Cut into squares while the filling is still firm from the freezer then leave to come to room temperature before serving. Keep any un-eaten squares in the fridge.