- Preheat the oven to 350°F. Line a 12-cup muffin tin with paper or silicone liners.
- Combine the milk and vinegar in a cup or small bowl. Set aside.
- Combine the oat flour, rice flour, cornstarch, baking powder, cinnamon, and salt in a large bowl and whisk until thoroughly combined.
- Combine the bananas, maple syrup, coconut oil, flax meal, and vanilla in a medium bowl and add the milk mixture. Stir until combined. Add the wet ingredients to the dry ingredients and stir together until combined. Fold in the blueberries and sugar.
- Pour the batter into the muffin tin. Bake for 23 to 25 minutes, until the tops are golden and firm. Let the muffins cool in the tin for about 5 minutes before transferring to a cooling rack. Cool completely before serving.
- Leftovers will keep in the fridge or at room temperature for 3 to 4 days.