01 - Preheat the oven to 350°F. Line a 12-cup muffin tin with paper liners or grease lightly with cooking spray.
02 - In a large bowl, whisk together oat flour, protein powder, sugar, baking powder, baking soda, salt, and cinnamon until well blended.
03 - In a separate bowl, whisk Greek yogurt, eggs, milk, melted coconut oil or butter, and vanilla extract until smooth and fully combined.
04 - Add wet ingredients to dry ingredients and mix gently until just combined. Do not overmix—some small lumps are acceptable.
05 - In a small bowl, mix together coconut sugar, cinnamon, and melted butter until a uniform paste forms.
06 - Fill each muffin cup halfway with batter. Add a small spoonful of the cinnamon swirl mixture to each cup. Top with remaining batter, then swirl gently with a toothpick to create marble patterns.
07 - Bake for 18-20 minutes, or until a toothpick inserted into the center of a muffin comes out clean.
08 - Let muffins cool in the tin for 5 minutes, then carefully transfer to a wire rack to cool completely.
09 - For icing, whisk powdered sugar with milk until a smooth, drizzling consistency is reached. Drizzle over cooled muffins before serving.