Adjust oven rack to the middle position. Preheat oven to 325°F.
Cover a 13x9-inch baking pan with two sheets of aluminum foil – the first sheet will go across the width and the second sheet should go the length of the pan. Cut the foil long enough to hang over by 3 inches so you can easily lift the cooked brownies from the pan. Spray the foil-lined pan with nonstick cooking spray.
In a medium bowl, combine the flour, salt, and baking powder. Whisk well.
1¼ cups gluten free flour, ½ tsp coarse kosher salt, ¾ tsp baking powder
Melt the butter in a large heatproof bowl. Add chopped chocolate and stir until completely smooth and incorporated. If necessary, gently heat the chocolate/butter mixture, on HIGH, in 10-second increments, until chocolate is melted. Stir often.
12 tbsp unsalted butter , 6 oz. unsweetened chocolate
When the chocolate mixture is smooth, gradually whisk in the sugar. Add eggs, one at a time, until thoroughly combined. Add vanilla and whisk again.
2¼ cups granulated sugar, 4 large eggs , 1 tbsp pure vanilla extract
Add flour mixture in three additions. Fold gently with a rubber spatula after each addition, until batter is smooth and fully incorporated. Stir in the chocolate chips.
1 cup semisweet chocolate chips
Transfer the batter to the prepared pan, spreading it into the corners and smoothing the surface.
Bake 30-35 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs attached.
Transfer brownies to a cooling rack and cool to room temperature. Remove cooled brownies from the pan by lifting the foil overhang. Cut into 2-inch squares and serve.