Preheat the oven to 350°F (175°C) and grease a standard muffin tin or line with silicone liners.
In a large mixing bowl, combine the rolled oats, ground cinnamon, baking powder, and salt.
In a medium bowl, whisk together the milk, egg, maple syrup, and vanilla extract until well emulsified.
Pour the liquid mixture into the dry ingredients and stir until the oats are thoroughly hydrated.
Fold the diced apples into the oatmeal mixture.
Distribute the mixture evenly into the muffin tin, filling each cup nearly to the top.
Bake for 20 to 25 minutes, or until the cups are firm to the touch and a toothpick inserted into the center comes out clean.
Allow the oatmeal cups to cool in the pan for 10 minutes before removing to a wire rack.