Preheat the oven to 350 degrees F.
Coat a muffin tin thoroughly with cooking spray. If you have silicone muffin cups, this would be a great place to use them!
Add the oats, baking powder, cinnamon, brown sugar, protein powder, applesauce, milk, eggs, butter, and vanilla to a large bowl.
Stir until well mixed.
Fold in your mix-ins of choice.
Spoon oatmeal mixture into the well-greased muffin tin until they are about ¾ full
Bake for 20-25 minutes until they are golden brown and set when touched gently.
Cool in the tin for about 5 minutes and then gently run a knife around the edge of each tin.
Cool for another 5 minutes and then gently loosen and remove each from the muffin tin.
Serve warm, or cool thoroughly before storing in the fridge or freezer.