01 - Preheat oven to 350°F. Line a 12-cup muffin tin with paper liners or grease lightly with cooking spray.
02 - In a large mixing bowl, whisk together oat flour, protein powder, coconut sugar, baking powder, baking soda, salt, and ground 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 incorporated.
04 - Pour wet ingredients into dry ingredients. Fold gently with a spatula until just combined—do not overmix or muffins will become dense.
05 - In a small bowl, stir together coconut sugar, cinnamon, and melted butter until mixture forms a crumbly paste.
06 - Divide batter evenly among 12 muffin cups, filling each halfway. Add a spoonful of cinnamon swirl mixture to each cup, then top with remaining batter. Swirl gently with a toothpick to distribute.
07 - Bake for 18-20 minutes until tops are golden and a toothpick inserted into the center comes out clean.
08 - Allow muffins to cool in the tin for 5 minutes, then transfer to a wire rack to cool completely.
09 - Whisk powdered sugar with 1-2 tablespoons milk until smooth drizzling consistency is reached. Drizzle over cooled muffins before serving.