Mash the ripe banana in a microwave-safe mug or small ramekin until liquid and smooth.

Stir in the melted butter, milk, and vanilla extract until fully incorporated.

Add the flour, brown sugar, baking powder, salt, and cinnamon to the mug.

Whisk the dry ingredients into the wet mixture until a smooth batter forms.

Microwave on high power for 90 to 120 seconds, or until the center is set and a toothpick comes out clean.

Allow the banana bread to cool for 2 minutes before consuming.