01 - Preheat oven to 350°F. Line a 12-cup muffin tin with paper liners or grease lightly.
02 - Whisk oat flour, protein powder, sugar, baking powder, baking soda, salt, and cinnamon in a large bowl.
03 - Whisk Greek yogurt, eggs, milk, melted coconut oil or butter, and vanilla extract in a separate bowl until smooth.
04 - Add wet ingredients to dry ingredients and mix gently until just combined. Do not overmix.
05 - Mix coconut sugar, cinnamon, and melted butter in a small bowl.
06 - Fill each muffin cup halfway with batter. Add a small spoonful of cinnamon swirl mixture. Top with remaining batter, then swirl gently with a toothpick.
07 - Bake for 18-20 minutes, or until a toothpick inserted into the center comes out clean.
08 - Cool in the tin for 5 minutes, then transfer to a wire rack. For optional icing, mix powdered sugar with milk until drizzling consistency is reached. Drizzle over cooled muffins before serving.