Preheat your oven to 350°F (175°C). Grease a 9x9-inch baking pan or line it with parchment paper.

In a large mixing bowl, whisk together gluten-free flour, cocoa powder, granulated sugar, brown sugar, and salt.

In another bowl, beat the eggs, then mix in melted butter and vanilla extract until smooth.

Pour the wet mixture into the dry ingredients, stir gently until combined, and fold in any mix-ins.

Transfer the batter to the prepared pan and bake for 25 to 30 minutes.

Let cool for about 10 minutes, then lift out using parchment paper and cool completely before slicing.