Put the biscuits into a bag, crush to crumbs, then melt the butter and stir into the biscuit crumbs until well mixed. Press the biscuit mixture into a 20cm/8in springform cake tin, letting some come a little way up the sides.
Melt the white chocolate in a pan of boiling water until melted then beat the cream cheese and double cream in a large bowl, until it is soft. Gently fold in the slightly cooled, melted white chocolate.
Smooth the top, cover the tin with cling film and refrigerate overnight before serving. When your ready to serve grate some white chocolate over the top for decoration.