Add the butter to the mug and microwave for about 20 to 30 seconds or until melted.
Add the mashed banana, milk, and vanilla extract, mixing thoroughly with a fork to combine.
Stir in the flour a little bit at a time, then mix in the remaining dry ingredients: sugar, baking powder, cinnamon, and salt.
Microwave for about 90 seconds.* This will take more or less time depending on your microwave's wattage. It is normal for the cake to rise above the rim of the mug as it cooks and sink as it cools. If the cake is still uncooked, microwave in 10-second increments.
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Notes
*This recipe was developed in an 1100-watt microwave. Times will vary. Higher-wattage microwaves may take less time while lower-wattage microwaves may take more time.