Preheat oven to 300°. Grease a 9- by 13-in. baking pan; set aside. Sift together flour, cocoa, salt, and baking powder into a bowl.
Microwave butter and bittersweet chocolate in a medium glass bowl, stirring often, until melted and smooth, about 1 1/2 minutes.
Whisk together sugar and eggs in a large bowl until blended. Add extracts and whisk to blend. Stir melted chocolate mixture into egg mixture. Gently fold in flour mixture with a plastic spatula until no streaks remain. Pour batter into prepared pan and spread evenly.
Bake brownies until a toothpick inserted 2 in. from edge comes out with a few crumbs sticking to it, 25 to 30 minutes. Cool to room temperature, about 45 minutes.
Put dark chocolate in a resealable plastic bag, snip off a small corner tip, and drizzle half over brownies. Sprinkle with candies and drizzle with remaining chocolate. Let cool until chocolate sets, 25 minutes, then cut into squares.
Note: Nutritional analysis is per brownie.