1. Prepare Your Baking Area: Preheat your oven to 375 F (190 C). Line a 12-cup muffin tin with paper liners or lightly grease it. This prevents sticking and makes cleanup easier.

2. Combine Dry Ingredients Thoroughly: In a large mixing bowl, whisk together the flour, protein powder, sugar, baking powder, baking soda, and salt. Use a whisk to ensure there are no lumps, especially from the protein powder, and that all ingredients are evenly distributed.

3. Mix Wet Ingredients Separately: In a medium bowl, whisk together the egg, milk, applesauce (or mashed banana), melted butter (or coconut oil), and vanilla extract until well combined and smooth.

4. Gently Combine Wet and Dry Mixtures: Pour the wet ingredients into the dry ingredients. Stir with a spatula or wooden spoon just until barely combined. It’s crucial not to overmix; a few lumps are perfectly fine. Overmixing can lead to tough baked goods.

5. Fold in Optional Mix-Ins (If Using): If you’re adding chocolate chips, berries, or nuts, gently fold them into the batter now. Distribute them evenly without overworking the batter.

6. Fill Muffin Cups: Divide the batter evenly among the 12 prepared muffin cups. Fill each cup about two-thirds full. This allows for proper rising without overflowing.

7. Bake Until Golden and Set: Bake for 20-25 minutes, or until a wooden skewer or toothpick inserted into the center of a muffin comes out clean. The tops should be golden brown and spring back lightly when touched.

8. Cool Before Enjoying: Remove the muffin tin from the oven and let the muffins cool in the tin for 5-10 minutes before transferring them to a wire rack to cool completely. This helps them set and prevents them from becoming soggy.