Preheat the oven to 350 degrees Fahrenheit (175 degrees Celsius) and coat a 6-count muffin tin with non-stick spray.
In a primary mixing bowl, combine oat flour, protein powder, baking powder, 1 tablespoon cinnamon, and salt.
In a secondary bowl, whisk together Greek yogurt, almond milk, egg, vanilla extract, and maple syrup until smooth.
Incorporate the wet mixture into the dry ingredients, stirring until just combined to maintain texture.
Combine the brown sugar substitute and the additional tablespoon of cinnamon in a small container for the swirl.
Distribute half of the batter into the muffin tins, sprinkle with the cinnamon-sugar mix, and top with the remaining batter.
Bake for 15 to 18 minutes until the internal structure is set and a tester comes out clean.
Whisk powdered sweetener with water to create a glaze and drizzle over the muffins once they have cooled for 5 minutes.