Directions

1. Preheat oven to 350 degrees. Coat a rimmed baking sheet with cooking spray. Line the bottom of baking sheet with parchment paper. Coat parchment with cooking spray; set aside. Whisk together flour, baking powder, and salt; set aside.
2. Heat butter and unsweetened chocolate in a medium heatproof bowl set over a pan of simmering water until almost melted. Remove from heat. Whisk until mixture is smooth and slightly cool.
3. Transfer chocolate mixture to the bowl of an electric mixer fitted with the whisk attachment. Add sugar. Beat on medium speed until smooth, about 3 minutes. Beat in eggs, 1 at a time. Add vanilla; beat 3 minutes more. Reduce speed to low. Gradually add flour mixture; beat until just combined. Stir in chocolate chunks.
4. Spread batter evenly into prepared pan. Bake until brownie has just set but still feels soft, about 20 minutes. Let cool on a wire rack. Cut into squares. Brownies can be stored in an airtight container at room temperature, up to 3 days.