How to Make It
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.
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.