Pour the butter into a lined 22x33cm baking dish, then sprinkle the biscuit crumbs on top, pressing down to form a crust. Sprinkle evenly with the chocolate chips, the butterscotch or white chocolate chips and desiccated coconut. Drizzle the condensed milk over the top, then sprinkle with the chopped walnuts.
Bake in the preheated oven until the condensed milk has thickened and turned brown, and the walnuts have browned, 30 to 45 minutes. Cool to room temperature on a wire rack and cut into squares to serve.