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, cocoa powder, baking powder, baking soda, and a dash of salt. Gradually fold the dry mixture into the wet mixture, ensuring even incorporation.
Chop the chocolate and dust in some flour and gently fold into the mix, 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 220°C (428°F). While the oven heats up, prepare 8-10 muffin liners in a muffin tin.
Distribute the muffin batter evenly among the prepared liners and sprinkle the reserved chocolate on top of each muffin.
Bake the muffins at 220c for approximately 5 minutes, then drop the temperature to 180c and bake for a further 18-20 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.