Coarsely chop walnuts using Chef's Knife. Place walnuts in 1-cup Prep Bowl; set aside. Combine flour, baking powder and salt in Small Batter Bowl. Cut butter into pieces and chop unsweetened chocolate; place butter and unsweetened chocolate into Classic Batter Bowl. Microwave, uncovered, on HIGH 1-2 minutes or until mostly melted, stirring every 30 seconds.

Add sugar and eggs to chocolate mixture; whisk until smooth using Stainless Whisk. Fold in flour mixture and 1/2 cup of the walnuts until incorporated using Small Mix 'N Scraper. Pour batter into Deep Covered Baker, spreading evenly. Microwave, uncovered on HIGH 5-7 minutes or until wooden pick inserted in center comes out clean and edges are firm. Carefully remove baker from microwave using Silicone Oven Mitts.

Meanwhile, whip marshmallows in half crosswise using Professional Shears. Top brownie with marshmallows, chocolate kisses and remaining 1/2 cup walnuts. Microwave, covered, on HIGH 1-2 minutes or just until marshmallows start to soften. Remove baker from microwave. Let stand, covered, 5 minutes. Spoon onto plates; serve with ice cream, if desired.

Oven directions: To bake in a conventional oven, prepare brownie batter as directed in Steps 1 and 2. Pour batter into baker. Bake, uncovered, at 350°F (180°C) 42–46 minutes or until wooden pick inserted into center comes out clean. Top with marshmallows, chocolate candies and remaining walnuts. Cover and bake an additional 5 minutes or until marshmallows start to soften. Let stand, covered, 5 minutes.