Vegetable-oil cooking spray
2 cups all-purpose flour
1 1/4 teaspoons baking powder
1 1/4 teaspoons salt
2 5/8 sticks unsalted butter (10 1/2 ounces), cut into small pieces
7 ounces unsweetened chocolate, coarsely chopped
2 3/4 cups sugar
4 large eggs
2 teaspoons pure vanilla extract
2 cups (10 ounces) white- or milk-chocolate chunks
- 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.
- 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.
- 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.
- 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.