Triple Chocolate Fudge

1 can condensed milk (not evaporated)
2 Tbsp butter
0.5 cup white chocolate chips
3 oz bar bittersweet chocolate
16 oz chocolate chips
0.5 cup chopped walnuts

Heat condensed milk and butter until butter melts. Off heat, stir in chocolate chips until melted. Stir in remaining ingredients. Spread in well-buttered 8 or 9 inch square pan and refrigerate until firm.