Fair warning: Our editors and test kitchen staff LOVED these gluten-free brownies. Words like "decadent" and "divine" were thrown around liberally.
Prepare the Brownies: Preheat oven to 350°F. Line a 9-inch square metal baking pan with parchment paper. Spray with baking spray.
Whisk together flour, cocoa, baking powder, and salt in a medium bowl. Microwave butter and chocolate in a microwavable bowl on MEDIUM (50% power) until melted, about 5 minutes, stirring every 30 seconds. Cool 5 minutes.
Beat sugar and eggs with a heavy-duty stand mixer fitted with a whisk attachment on high speed until light and fluffy, 2 to 3 minutes. Reduce speed to low. Add flour mixture alternately with cooled melted chocolate mixture, beginning and ending with flour mixture. Beat until fully incorporated.
Spread brownie batter in prepared pan. Bake in preheated oven until moist crumbs cling to a toothpick inserted in center, 40 to 45 minutes. Cool in pan on a wire rack 15 minutes.
Prepare the Glaze: Microwave chocolate, butter, and corn syrup in a microwavable bowl on MEDIUM (50% power) until melted, about 5 minutes, stirring every 30 seconds. Whisk in powdered sugar. Spread over brownies; top with chopped hazelnuts. Let cool completely. Cut into 16 squares.