Preheat your oven to 350°F (177°) then line a 12-cup muffin pan with paper or silicone muffin liners.

Whisk the brown rice flour, oat flour, potato starch, tapioca starch, baking powder, baking soda, psyllium husk powder, kosher salt, and cinnamon in a medium bowl until combined. Set it aside.

Whisk the vegetable oil, granulated sugar, egg, sourdough discard, and whole milk in a large bowl until combined.

Next, add the gluten-free flour mixture to the wet ingredients and mix until combined.

Scoop the cinnamon muffin batter into the prepared pan using a 3-tablespoon cookie scoop to fill each muffin liner 3/4 full

Bake the cinnamon muffins for 20-25 minutes until a toothpick inserted into the center of a cinnamon muffin comes out clean.

Let the cinnamon muffins cool for 5 minutes in the pan, then remove them from the silicone liners, if using, and place them on a wire rack to cool. If you used paper liners, transfer the muffins from the pan to the cooling rack.

Dust with confectioner's sugar, if desired.