Spray two 12-ounce microwave-safe mugs lightly with nonstick cooking spray. Set aside.
In a medium bowl, combine the mashed bananas, brown sugar, melted butter, milk, and vanilla extract, stirring until smooth.
Add the all-purpose flour, baking powder, cinnamon, and salt, stirring just until combined. Do not overmix.
Pour the batter evenly between the two prepared mugs.
Top each mug with 1 teaspoon butter, then sprinkle with granulated sugar and cinnamon.
Microwave each mug separately for 90 to 120 seconds. Check for doneness by ensuring the center is set. If needed, continue microwaving in 10-second intervals until cooked through.
Serve warm.