Heat oven to 350°F. Line 13x9-inch pan with foil so foil extends over sides of pan. Spray foil with nonstick cooking spray. In large bowl, combine all brownie ingredients; stir until blended. Spread batter in sprayed foil-lined pan.
Bake at 350°F. for 28 to 30 minutes or until set. DO NOT OVERBAKE. Cool 45 minutes or until completely cooled. Cover with foil; freeze brownies 30 minutes.
Remove foil cover. Remove brownies from pan by lifting foil; place on cutting board. With 2 1/2-inch tree-shaped cookie cutter, cut out 15 brownie trees. Place on wire rack set over waxed paper.
Place frosting in microwave-safe measuring cup with pouring spout. Microwave on HIGH for 20 to 40 seconds or until pourable, stirring occasionally. DO NOT BOIL. Carefully pour melted frosting over tops of brownie trees. Immediately sprinkle with candy sprinkles.