Adjust the oven rack to the middle position and preheat the oven to 325 degrees F.

Line a 13×9-inch baking pan with two sheets of aluminum foil: place the first sheet across the pan one way and the second sheet perpendicular to it so they overlap. Cut the foil so it hangs about 3 inches over all edges. Smooth the foil and spray the foil-lined pan with nonstick cooking spray.

In a medium bowl, whisk together the gluten free flour, coarse kosher salt, and baking powder until combined.

Put the unsalted butter in a large heatproof bowl and add the finely chopped unsweetened chocolate. Stir and heat as needed in the microwave on HIGH in 10-second increments, stirring between each, until the mixture is completely smooth and melted.

Gradually whisk the granulated sugar into the warm chocolate/butter mixture until incorporated.

Add the eggs one at a time, whisking after each addition until fully combined. Whisk in the vanilla extract.

Add the flour mixture to the chocolate mixture in three additions, folding gently with a rubber spatula after each addition until the batter is smooth and evenly combined. Stir in the semisweet chocolate chips.

Transfer the batter to the prepared pan, spreading it into the corners and smoothing the surface.

Bake for 30–35 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs attached (not wet batter).

Remove the pan from the oven and place it on a cooling rack. Cool the brownies completely to room temperature.

Use the foil overhang to lift the cooled brownies from the pan, remove the foil, and cut into 2-inch squares.

Serve immediately or store in an airtight container at room temperature for up to 3 days.