01 - Preheat the oven to 350°F. Grease a 12-cup muffin tin or line with paper liners.
02 - In a large bowl, mix rolled oats, ground cinnamon, baking powder, and salt until evenly distributed.
03 - In a separate bowl, whisk together eggs, milk, maple syrup or honey, melted butter or coconut oil, and vanilla extract.
04 - Pour wet ingredients into dry ingredients and stir gently until just combined.
05 - Fold in diced apples and any optional nuts or dried fruit delicately to avoid overmixing.
06 - Divide the mixture evenly among prepared muffin cups, filling each nearly to the rim.
07 - Bake for 25 to 28 minutes until the cups are set and lightly golden on top.
08 - Let the oatmeal cups cool in the pan for 5 minutes before transferring to a wire rack to cool completely.