Preheat oven to 350°F (175°C) and line a 6-cup muffin tin with silicone liners.
In a large mixing bowl, combine the protein powder, oat flour, baking powder, 1 tsp cinnamon, salt, and sweetener.
In a separate bowl, whisk together the Greek yogurt, egg white, almond milk, and vanilla extract until smooth.
Merge the wet ingredients into the dry ingredients, stirring until a thick batter forms.
Prepare the swirl by mixing 1 tbsp cinnamon, brown erythritol, and melted coconut oil in a small ramekin.
Distribute half of the batter evenly across the 6 muffin cups.
Spoon half of the cinnamon swirl mixture over the batter in each cup, then layer the remaining batter on top.
Finish by adding the remaining cinnamon swirl mixture to the tops and using a toothpick to create a marbled pattern.
Bake for 18 to 22 minutes or until the center is firm to the touch.
Let the muffins cool in the tin for 10 minutes before transferring to a wire rack.