Place a large heatproof bowl over a pan of simmering water. Do not allow the water to touch the bottom of the bowl.
Add the white chocolate and condensed milk. Gently melt together, stirring occasionally.
Place the cookies in a bag and crush with a rolling pin to form coarse crumbs and some bigger chunks. Stir 3/4 of these into the white chocolate mixture.
Spoon into a lined square baking tin. Smooth down with the back of a spoon. Sprinkle the remaining cookie pieces on top and lightly press down ( if you like, drizzle some melted dark chocolate over now). Set for an hour in the fridge.