In a microwave-safe mug, combine the flour, granulated sugar, ground cinnamon, and baking powder. Mix well.
Add the milk, melted butter, and vanilla extract to the dry ingredients. Stir until the batter is smooth and fully combined.
In a small bowl, mix the brown sugar, ground cinnamon, and melted butter for the topping.
Spoon the topping mixture over the batter in the mug, gently swirling it into the batter with a fork or knife.
Microwave the mug on high for 40-60 seconds, or until the coffee cake is set and springs back lightly when touched. Cooking time may vary depending on your microwave.
Allow the coffee cake to cool for a minute before enjoying.
Notes
For a richer flavor, you can add a pinch of nutmeg or use almond extract instead of vanilla.