Heat oven to 350°F. Line a 9-by-13-inch pan with a strip of foil or parchment, allowing edged to hang over opposite sides.
Melt butter and chocolate in a microwave or double boiler; cool slightly. In a bowl, whisk together eggs, salt, sugars and vanilla; whisk in melted chocolate. Fold in flour until just combined. Stir in walnuts, if using.
Spread batter in an even layer in pan. Bake until toothpick inserted in the center comes out with a few crumbs, 20 to 22 min (don't overbake). Cool completely in pan.
Make topping and spread over brownies; let set at room temperature, 30 min. Lift brownies from pan using the parchment overhang. Lightly coat a knife with cooking spray, then trim ¼ inch off crisp edges of brownies to make clean sides. Cut brownies into squares, cleaning knife after every cut. Sprinkle candies over top.
Make this when you're ready to frost cooled brownies. In a heat-proof bowl over a pot of simmering water, combine 1 cup sugar, 4 egg whites and a pinch of salt. Whisk until sugar is dissolved completely and mixture is hot, about 5 min. Remove from heat, add ¼ tsp peppermint extract and 3 to 4 drops red food coloring and whip with electric mixer until mixture holds firm peaks, about 9 min.
You could certainly skip the homemade brownie part of this recipe and use a box mix like my other fun brownie treats. I find box mix brownies to be nice and dense and chewy. This homemade version has more chocolate flavor and is more fudgy than chewy.