MICROWAVE chocolate and butter in large microwaveable bowl on HIGH 2 min. or until butter is melted. Stir until chocolate is completely melted; cool slightly.
SPRINKLE 2 Tbsp. sugar onto large sheet waxed paper. Combine marshmallows and nuts in large bowl. Add chocolate mixture; toss to coat. Divide mixture in half; place on prepared waxed paper. Shape each half into 8-inch log; sprinkle with remaining sugar.
REFRIGERATE 2 hours or until chilled. Cut each log into 8 slices.
