Heat the oven to 180C/fan 160C/gas 4. Butter a 23cm springform tin
and line the base and sides with baking paper. To make the base, bash the biscuits in a bowl with the end of a rolling pin to a fine crumb, then stir in the melted butter, milk powder and a pinch of salt. Press the mixture into the base of the tin, pressing it down and smoothing with the back of a spoon. Bake for 10 minutes, then leave to cool.