In a mixing bowl, combine the melted light butter, granulated sweetener and vanilla extract. Mix until the ingredients are well combined and the mixture has a creamy texture.
Add the egg, yogurt, and buttermilk (prepared by combining milk with the juice of 1/4 lemon). Mix thoroughly until the wet ingredients are fully integrated.
Sift in the flour, protein powder, baking powder, baking soda, cinnamon and a dash of salt. Then add the shredded and chopped apple and mix and fold the mixture until just incorporated.
OPTIONAL: 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.
Mix the dry crumble ingredients and add the butter and 1 tsp of milk. Mix with your hands until a crumble that resembles wet sand forms.
Preheat your oven to 220°C (428°F). While the oven heats up, prepare 8 muffin liners in a muffin tin.
Distribute the muffin batter evenly among the prepared liners. For optimal results add 8 muffins liners to 2x 6 muffin tin trays, filling up the ones without liners with water. (4 muffins per tray).
Top each muffin with a generous sprinkle of the crumble mixture.
Bake the muffins at 220c for approximately 5 minutes, then drop the temperature to 180c and bake for a further 20-25 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.