Preheat the oven to 350 degrees.
Line a muffin tin with cupcake liners.
In a large bowl combine the butter and brown sugar and mix well. Add in the eggs and mix until combined.
Stir in the vanilla and bananas. Mix well.
In a medium bowl combine the flour, oats, baking soda, and salt. Mix well.
Add half the flour mixture to the banana mixture and mix until combined.
Stir in the milk and mix well.
Add the remaining flour mixture and mix until combined.
Pour the batter into the prepared muffin cups filling 3/4 of the way full.
Bake for 20-22 minutes or until a toothpick inserted into the center comes out clean. Cool completely on a wire rack.
To make the glaze mix together the butter and vanilla extract. Stir in the cocoa powder until combined.
Add the powdered sugar a half cup at a time, stirring after each addition, until desired consistency is reach.
Spread the glaze on top of each muffin.