Preheat oven to 350°. Butter a 9- by 13-in. baking pan. In a medium saucepan, stir butter and bittersweet chocolate over very low heat with a heatproof rubber spatula until just melted (do not let simmer or boil). Remove from heat and let cool slightly, about 5 minutes.
In a large bowl, whisk together eggs, sugar, vanilla, and salt. Slowly pour chocolate-butter mixture into egg mixture, whisking constantly. Into a separate bowl, sift flour, cocoa powder, and baking powder, then whisk together. With a clean rubber spatula, gently fold flour mixture into egg mixture until just combined.
Spread batter evenly in the baking pan. Bake 20 to 30 minutes, or until brownies are firm, beginning to pull away from sides of pan, and a toothpick inserted in center emerges with only a few crumbs clinging to it. Let brownies cool completely in the pan on a wire rack.
Reduce oven temperature to 300°. Line a level, rimmed 11 1/2- by 16-in. baking sheet with parchment. With a pencil, draw a grid of 1 3/4-in. squares on parchment (5 rows of 8), leaving about a 1-in. border, and turn parchment over. Place a mint in the center of each square. Bake 15 to 25 minutes, or until completely melted into 1 large sheet, but remove before candy begins to turn yellow. Immediately score peppermint squares with a pizza cutter, using pencil lines as your guide. If scored lines don't hold, rescore quickly. Let candy cool completely. Gently break into squares along score lines. Arrange, bake, and score remaining 40 candies the same way (you'll have extra squares; pick the prettiest).
In the top of a double boiler or a metal bowl set over simmering water, combine chopped semisweet chocolate and whipping cream. Cook, stirring often, until melted and blended. Remove from heat and let ganache cool until just warm.
Pour ganache over cooled brownies in pan and allow to set until firm, about 2 hours at room temperature or 1 hour in the refrigerator.
Place peppermint squares bottom side up on ganache, leaving enough room between squares to cut brownies. Cut brownies into squares along edges of peppermint tops, wiping knife clean between cuts. Remove brownies from pan, discard (or eat) scraps, and return brownies to pan. Cover with plastic wrap, making sure wrap does not touch peppermint, and refrigerate 3 hours (to soften candy a bit). Serve slightly chilled.
More toppings to try. Why stop at peppermint? Try these other embellishments for our rich, soft brownies. Be generous with amounts, and chill brownies so that the topping sets.
Coconut and Macadamia: Sprinkle shredded toasted coconut and chopped toasted macadamia nuts over the just-warm ganache layer.
Crushed Toffee: Sprinkle crushed toffee bits over the just-warm ganache layer.
Double Peanut: Warm up natural-style peanut butter (creamy or chunky) and spread over cooled brownies. Refrigerate until peanut butter is firm; top with ganache and chopped peanuts.
Raspberry: Spread raspberry jam over cooled brownies and top with ganache.
Rocky Road: Preheat broiler. Spread half of the just-warm ganache over cooled brownies, then top with mini marshmallows and chopped walnuts. Broil brownies for about 1 minute, or until marshmallows are toasted and beginning to melt together. Remove from oven and drizzle with remaining ganache.
White Chocolate Swirl: Spoon dollops of melted white chocolate over the just-warm ganache layer and make marbled swirls using a toothpick or a knife.
Note: Nutritional analysis is per brownie. Notes: You can make the brownies with ganache 3 days ahead, cover with plastic wrap (making sure it does not touch ganache), and chill. The tops can be made up to a week ahead and stored airtight at room temperature. Look for hard, round peppermint swirl candies, also called starlight mints, with bright red color. Keep an eye on the candies as they melt; some brands don't spread as quickly or as thinly as others. You may want to make a test batch using 12 mints arranged in 3 rows of 4 to determine whether you'll need to adjust your oven's temperature or space the candies closer together.