In a microwave-safe mug, combine the all-purpose flour, granulated sugar, baking powder, and salt. Stir well to ensure there are no lumps.
Add the melted butter, milk, and vanilla extract to the dry ingredients in the mug. Mix thoroughly until a smooth batter forms, scraping the bottom of the mug to incorporate all ingredients.
Sprinkle a pinch of cinnamon sugar on top of the batter, stirring lightly to incorporate if desired.
Microwave the mug on high for about 1 minute and 30 seconds. The exact time may vary depending on your microwave's wattage, so start checking for doneness around the 1-minute mark. The cake should rise and be set in the middle.
Let the mug cake cool for a couple of minutes before serving. Optionally, sprinkle more cinnamon sugar on top before enjoying.