PREPARATION:

1. Line 8-inch square pan with foil, extending 1-inch over ends of pan. Lightly butter foil.
2. Melt chocolates in medium saucepan over low heat, stirring constantly. Remove from heat. Stir in condensed milk; add marshmallows and walnuts, if desired; stir until blended.
3. Spread chocolate mixture evenly in prepared pan. Score into 2-inch triangles by cutting halfway through fudge with sharp knife while fudge is still warm.
4. Refrigerate until firm. Remove from pan by lifting fudge and foil. Place on cutting board; cut along score lines into triangles. Remove foil. Store in airtight container in refrigerator.