In heavy saucepan over low heat, melt chocolate with condensed milk and butter. (Do not substitute evaporated milk for condensed.) Remove from heat. In large bowl, combine nuts and marshmallows. (You may prefer plain unsalted peanuts or walnuts). Fold in chocolate mixture. Spread in 9 by 13 inch pan lined with wax paper. (Or drop teaspoons on to baking sheets lined with wax paper.) Chill 2 hours or till firm. Remove from pan, peel off paper, cut into squares. Cover and store at room temperature. MAKES: 40 SQUARES