Line a 12-cup muffin pan with silicone liners or grease it well.

Mix the oats, cinnamon, baking powder, and salt in a large bowl.

Whisk the milk, egg, maple syrup, and vanilla in a separate bowl.

Stir the wet mixture into the dry ingredients, then fold in the apple.

Divide the mixture among the cups.

Cover and freeze until firm. Store in a freezer bag for up to 2 months.

Thaw overnight in the refrigerator and warm before serving.