01 - Preheat the oven to 350°F. Grease a 12-cup muffin tin or line it with paper liners.
02 - In a large bowl, stir together rolled oats, ground cinnamon, baking powder, and salt until evenly mixed.
03 - Whisk eggs, milk, maple syrup or honey, melted butter or coconut oil, and vanilla extract in a separate bowl until smooth.
04 - Add the wet mixture to the dry ingredients, stirring just until combined without overmixing.
05 - Gently fold diced apples and any desired nuts or dried fruit into the batter.
06 - Divide the batter evenly among the 12 muffin cups, filling each nearly to the top.
07 - Bake for 25 to 28 minutes, until the cups are set and light golden brown.
08 - Let the cups cool in the pan for 5 minutes before transferring to a wire rack to cool completely.