Preheat the oven to 350°F (175°C). Line a 12-cup muffin tin with paper liners or grease lightly.
In a large bowl, whisk the applesauce, grated apple, milk, oil or butter, egg, vanilla, and maple syrup or honey until well combined.
In a separate bowl, stir together the oats, flour, cinnamon, baking powder, baking soda, and salt.
This helps distribute the leaveners evenly.
Add the dry ingredients to the wet ingredients. Stir gently with a spatula until just combined. The batter will be thick.
Avoid overmixing to keep the muffins tender.
Fold in any optional add-ins, keeping pieces small for toddler-friendly bites.
Divide the batter evenly among the muffin cups. Fill each about 3/4 full.
Bake for 15–18 minutes, until the tops are set and a toothpick comes out with a few moist crumbs.
Cool in the pan for 5 minutes, then transfer to a rack. Serve slightly warm or at room temperature.