Preheat oven to 350°F (175°C) and spray a 12-count muffin tin with non-stick cooking spray.
In a large mixing bowl, combine oat flour, vanilla protein powder, granulated sweetener, and baking powder.
Whisk in the Greek yogurt, eggs, almond milk, and vanilla extract until a smooth batter forms.
In a small bowl, whisk together the cinnamon and melted coconut oil to create the swirl mixture.
Divide half of the batter among the muffin cups, drop a teaspoon of the cinnamon mixture into each, then top with the remaining batter.
Bake for 20 minutes or until a toothpick inserted into the center comes out clean.
Allow muffins to cool for 5 minutes before removing from the tin.