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.