01 - Preheat oven to 350°F and line a 12-cup muffin tin with paper liners or lightly grease with nonstick spray.
02 - In a large bowl, whisk together oat flour, protein powder, baking powder, baking soda, salt, and cinnamon until evenly distributed.
03 - In a separate bowl, whisk eggs, almond milk, Greek yogurt, maple syrup, melted coconut oil, and vanilla extract until smooth.
04 - Add wet ingredients to dry ingredients and fold gently until just combined, avoiding overmixing to ensure a tender texture.
05 - Mix brown sugar, ground cinnamon, and melted coconut oil or butter in a small bowl to create the cinnamon swirl mixture.
06 - Fill each muffin cup halfway with batter, add a spoonful of the cinnamon swirl mixture, then top with remaining batter and swirl gently with a toothpick or knife.
07 - Bake in preheated oven for 16 to 18 minutes until a toothpick inserted in the center comes out clean.
08 - Allow muffins to cool in the tin for 5 minutes before transferring to a wire rack to cool completely.
09 - If desired, combine powdered sugar and milk, then drizzle over cooled muffins before serving.