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 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 creamy.
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, mix coconut sugar, cinnamon, and melted butter until combined into a crumbly paste.
06 - Fill each muffin cup halfway with batter. Add a small spoonful of cinnamon swirl mixture to each cup. Top with remaining batter, then gently swirl with a toothpick to marble the cinnamon throughout.
07 - Bake for 18-20 minutes until a toothpick inserted into the center comes out clean and muffins are golden on top.
08 - Let muffins cool in the tin for 5 minutes, then transfer to a wire rack to cool completely. If desired, whisk powdered sugar with milk to drizzling consistency and drizzle over cooled muffins.