Coarsely chop walnuts using Chef's Knife. Place walnuts in (1-cup/250-mL) 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 (125 mL) 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, snip marshmallows in half crosswise using Professional Shears. Top brownie with marshmallows, chocolate kisses and remaining 1/2 cup (125 mL) walnuts. Microwave, covered, on HIGH 1–-2 minutes or until marshmallows start to soften. Remove baker from microwave. Let stand, covered, 5 minutes. Spoon onto plates; serve with ice cream, if desired.