1. Preheat Oven and Prepare Muffin Pan: Start by preheating your oven to 375 F (190 C). Line a 12-cup muffin tin with paper liners or lightly grease each cup with cooking spray. This prevents sticking and makes cleanup easier.

2. Combine Dry Ingredients: In a large mixing bowl, whisk together the all-purpose flour, protein powder, granulated sugar, baking powder, and salt until they are well combined. Ensure there are no lumps, especially from the protein powder.

3. Mix Wet Ingredients: In a separate medium bowl, whisk together the milk, unsweetened applesauce, vegetable oil, egg, and vanilla extract until the mixture is smooth and uniform.

4. Combine Wet and Dry Mixtures: Pour the wet ingredients into the bowl with the dry ingredients. Stir gently with a spatula or wooden spoon just until everything is combined. Be careful not to overmix; a few small lumps are perfectly fine and will result in tender muffins. Overmixing can lead to tough muffins.

5. Fold in Optional Mix-ins: If you’re using chocolate chips, chopped nuts, or other additions, gently fold them into the batter until they are evenly distributed.

6. Fill Muffin Cups: Divide the batter evenly among the 12 prepared muffin cups. Each cup should be about two-thirds full.

7. Bake Until Golden: Place the muffin tin in the preheated oven and bake for 18-22 minutes, or until a wooden skewer or toothpick inserted into the center of a muffin comes out clean. The tops should be lightly golden brown.

8. Cool and Serve: Once baked, remove the muffin tin from the oven and let the muffins cool in the pan for about 5 minutes before transferring them to a wire rack to cool completely. This helps prevent them from getting soggy.