Preheat oven to 375 degrees and line a 24-cup mini muffin pan with paper liners or spray with non-stick spray.

In a medium bowl, whisk together the egg, milk, avocado oil, and vanilla extract.

In a large bowl, whisk together the flour, xanthan gum, brown sugar, baking powder, cinnamon, and salt. Stir to combine. If you using fresh blueberries, stir them in to coat with the flour. Pour in the wet ingredients and stir until all the dry ingredients are incorporated. (If using frozen blueberries, stir them in AFTER the wet ingredients are mixed into the dry ingredients.)

Divide the batter evenly among the 24 mini muffin cups and bake at 375 degrees for 12 minutes or until done in the center. Cool in the pan for 3-5 minutes before serving. Serve hot or room temperature. Leftover muffins can be frozen or stored in an air-tight container up to 2 days.