Lightly butter an 8-by-8-inch or 9-by-9-inch glass baking dish. In a medium saucepan over medium-low heat, melt butter and chocolate, stirring frequently. Remove from heat. Stir in peanut butter until smooth. Stir in chopped nuts. Let cool 10 to 15 minutes. Gently stir in marshmallows (they shouldn't melt). Pour fudge into prepared pan and refrigerate several hours.