Preheat your oven to 350°F / 177°C. Prepare an 8x8 baking pan (I like glass) with butter and line with parchment paper. Set aside.
In a medium bowl, combine the white rice flour, brown rice flour, tapioca starch, xanthan gum, milk powder, and salt. Stir to combine and set aside.
In a medium sauce pan, begin to melt your butter and chocolate together over a low flame. You want them to come together but do not bring to a boil.
Remove from the heat and stir in your sugar till it is fully incorporated. Add the eggs and vanilla, stirring constantly to make sure the eggs don't cook. The sugar should have cooled down enough so that shouldn't happen.
Now add your flour mixture to the wet ingredients in two batches, combing well before adding the second. When it is fully incorporated, you can add in any mix-ins (if using).
Transfer to your prepared baking pan. Place in the oven and cook for 30 - 35 minutes or until a toothpick inserted comes out (almost) clean. You don't want to over bake these so keep an eye on them!
Let cool completely before cutting and enjoying. These can be stored on the countertop for 3 days in an air tight container, or in the fridge for up to 5 days. These can be frozen for up to 3 months.