Toss together oats, baking powder, cinnamon, salt, and brown sugar in a bowl.
In another bowl, whisk together the milk, egg and vanilla.
Pour the wet mixture over the oats and stir to combine.
Divide evenly between wells of a muffin tin, lined with cupcake papers or sprayed with nonstick spray.
Bake at 350° for 25-30 minutes or until set.
Notes
This can be made ahead and reheated in the microwave. To freeze, place cooked cups on a baking sheet in the freezer for two hours and then transfer to a ziplock bag. Thaw in refrigerator overnight.