Preheat oven to 350°F and line a 12-cup muffin tin liners (silicone are my favorite but paper ones are fine as well).
Mix together dry ingredients: oats, cinnamon, baking powder and salt.
In a separate bowl, whisk together milk, eggs, banana, and vanilla.
Pour wet ingredients into dry ingredients. Stir in raisins.
Spoon oat mixture evenly into 12 muffin cups.
Sprinkle brown sugar evenly on top of muffins.
Bake for 30 minutes, or until a toothpick comes out clean.
Store in a container in the fridge or freeze in a large ziplock.