Preheat the oven to 375°F. Lightly grease a 9-inch square pan and line it with parchment if you wish.
Place 1 cup gluten free flour, ¾ cup unsweetened cocoa powder, 1 ½ cups granulated sugar, ½ teaspoon fine sea salt, ¼ teaspoon baking powder, 3 large eggs, 8 tablespoons unsalted butter, and ¼ cup vegetable oil in a large bowl, then stir and mix well to create a smooth batter.
Spoon the batter into the prepared pan.
Bake the brownies for about 26-28 minutes; a toothpick inserted into the center should come out clean or with a few moist crumbs.
Notes
CN's Brownie Tips
Don't overbake - Gluten-free bakes have a tendency to get dry, so make sure you check the brownies at around 26 minutes. A few moist crumbs on the tester is fine, and they probably won't go past 28 minutes of baking time, so check them again soon if they're not quite done.
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 do use a metal pan, check at 25 minutes.
What size? I baked this batter in a 9-inch pan, and an 8-inch pan will also work.
Rest the brownies after baking - Let the brownies cool in the pan for at least 10 minutes after you pull them from the oven so that the gluten-free crumb sets nicely and it's easier to cut them.
Omit the baking powder - I love the slight cakiness of these brownies, but you like a more fudgy batch of brownies, skip adding the baking powder.
Dairy-free - Swap melted dairy-free butter or a neutral oil of choice for the butter.
Refined sugar-free - Coconut sugar instead of granulated sugar works nicely here.