In a mixing bowl, combine the melted light butter and granulated sweetener. Mix until the ingredients are well combined and the mixture has a creamy texture.
Add the egg, yogurt, buttermilk (prepared by combining milk with vinegar or lemon juice), and vanilla extract to the butter mixture. Mix thoroughly until the wet ingredients are fully integrated.
Sift in the flour, protein powder, cinnamon, baking powder, baking soda, and a dash of salt. Gradually fold the dry mixture into the wet mixture, ensuring even incorporation.
Dust the blueberries in some flour and gently fold into the mix, along with the rolled oats, reserving a small portion for topping the muffins.
Rest and Develop Flavor and texture: For optimal flavor development and texture, cover the muffin batter and refrigerate for a minimum of 30 minutes. This step, while optional, allows the ingredients to harmonize and enhances the taste profile of the muffins.
Preheat and Line: Preheat your oven to 180°C (350°F). While the oven heats up, prepare 12 muffin liners in a muffin tin. Distribute the muffin batter evenly among the prepared liners and sprinkle the reserved blueberries on top.
Bake for 20-15 minutes or until a toothpick inserted into the center emerges clean.
Once baked, allow the muffins to cool in the tin for 10-15 minutes, then transfer the muffins to a wire rack for further cooling.