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 evenly distributed.
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 and fold gently with a spatula until just combined. Do not overmix—small lumps are acceptable.
05 - In a small bowl, combine coconut sugar, cinnamon, and melted butter. Stir until mixture forms a crumbly paste.
06 - Fill each muffin cup halfway with batter. Add a small spoonful of cinnamon swirl mixture to each cup, then top with remaining batter. Swirl gently with a toothpick to distribute the cinnamon mixture.
07 - Bake for 18-20 minutes until a toothpick inserted into the center comes out clean or with just a few moist crumbs.
08 - Let muffins cool in the tin for 5 minutes, then transfer to a wire rack to cool completely.
09 - If desired, whisk powdered sugar with milk until drizzling consistency is reached. Drizzle over cooled muffins before serving.