Preheat and Prepare: Preheat your oven to 350°F (175°C). Grease a 12-count donut pan well.
Combine Dry Ingredients: In a large bowl, whisk together the gluten-free flour blend, sugar, baking powder, salt, and cinnamon (if using).
Combine Wet Ingredients: In a separate medium bowl, whisk together the milk, melted butter, egg, and vanilla extract.
Combine Wet and Dry: Pour the wet ingredients into the dry ingredients and mix until just combined. Be careful not to overmix. The batter should be thick but pourable.
Fill the Donut Pan: Spoon or pipe the batter into the prepared donut pan, filling each mold about two-thirds full.
Bake: Bake for 10-12 minutes, or until a toothpick inserted into the center comes out clean.
Cool: Let the donuts cool in the pan for a few minutes before transferring them to a wire rack to cool completely.