Start by preheating your oven to 375°F and lining a muffin tin with liners. If you’re skipping liners, spray the tin well so the muffins pop right out.
While the oven heats up, dice your apples and set them aside.
Next, mash the bananas on a plate with a fork until they’re a smooth paste. I know it sounds fussy, but a flat surface makes it so much easier to get every lump out than mashing in a bowl.
Scrape the bananas into a large bowl, add the cooled melted butter, and whisk until combined. Then add the egg, maple syrup, and vanilla, and whisk again.
Add the flour, protein powder, cinnamon, baking powder, and salt right on top of the wet ingredients. Stir with a spatula just until you can’t see any dry flour, and then stop.
Gently fold in the diced apples with as few strokes as possible.
Scoop the batter into your muffin cups (nine for big muffins, twelve for regular) and bake for 18-25 minutes. Ovens vary, so start checking early. When a toothpick in the center of a muffin comes out clean, they’re done, and mine are always ready right around the 19-20 minute mark.
Let them cool in the tin for a few minutes, then move them to a wire rack.