Line a 12-cup muffin tin with paper liners. Melt butter in a small saucepan over medium. Stir in peanut butter, graham cracker crumbs, and maple sugar; mix well. Remove from heat. Divide mixture evenly among muffin cups, about 2 Tbsp per cup. Combine chocolate and milk in another saucepan. Stir over medium until chocolate has melted. Spoon evenly over peanut butter mixture. Top with chopped nuts. Refrigerate until set, about 2 hours.