Preheat the oven to 350°F. Line a 9-inch square baking pan with aluminum foil. Spray the foil with cooking spray. In a medium bowl, whisk together the cocoa powder, gluten-free flour, baking powder, and salt.
Melt the butter in a medium saucepan over medium heat until completely melted and starting to bubble. Pour in the sugar and vigorously whisk for 1 minute to combine. Remove from the heat.
Add the eggs to a large bowl. Use a hand mixer to whip the eggs for 1 minute, until starting to foam.
With the mixer running on low, slowly add the butter/sugar mixture. Once the butter/sugar mixture has been added, increase the mixer speed to medium and whip for 1 minute. Stir in the vanilla.
Using a rubber spatula, stir in the dry ingredients until just combined. Fold in the chocolate chips, saving 1/4 cup to sprinkle over the top.
Spread the batter into the prepared pan. Sprinkle with the remaining chocolate chips. Bake for 35-40 minutes, until a toothpick inserted into the center comes out with a few moist crumbs.
Remove from the oven and let cook on a wire rack for at least 30 minutes before slicing. I like to press a few more chocolate chips on top. Enjoy!