Preheat the oven to 350°F. Lightly grease a 9" x 13" pan.
In a saucepan set over low heat, combine the 16 tablespoons unsalted butter and 2 ¼ cups granulated sugar, and heat, stirring, until the butter is melted, hot, and shiny.
Place the 4 large eggs in a large bowl with the 1 ¼ cups Dutch-process cocoa, 1 teaspoon fine sea salt, and 1 teaspoon baking powder, and, using an electric mixer, beat them at medium speed until smooth. Mix in the butter/sugar mix, then stir in the 1 ½ cups gluten free flour until smooth.
Pour the batter into the prepared pan, let it rest for 15-20 minutes, then bake for 28 to 32 minutes, until a toothpick inserted into the center comes out clean, or with a few moist crumbs, and the edges are set. Remove from the oven and cool on a rack before cutting and serving. You can do this straight from the pan if you like.
Notes
CN's variation ideas and tips
Add chocolate - Stir in milk, semisweet, or white chocolate chips, or chop up your favorite chocolate bar.
Or candy - Take gluten free brownies to the next level by adding your fav (gluten-free) candy bar! I love chopping up Reese's peanut butter cups or stirring in Reese's pieces or M&Ms to make them extra decadent.
Stir in nuts - Nuts are a classic addition to brownies. Try chopped walnuts, peanuts, or almonds.
And coconut - If you're a coconut fan, mix in 1 cup of sweetened or unsweetened shredded coconut!
Don't overbake - Gluten-free bakes have a tendency to get dry, so make sure you check the brownies at 30 minutes. A few moist crumbs on the tester is fine - this is better than overbaking (and drying out) your brownies!
The pan matters - I like to bake brownies in a glass or ceramic pan. This distributes the heat better so that they bake more evenly. A metal pan will bake the brownies more quickly, so if you use a metal pan, check at 25 minutes for doneness.
Rest the brownies after baking - Let the brownies cool in the pan at least 10 minutes after you pull them from the oven so that the gluten-free crumb sets up nicely and it's easier to slice them up.