In a microwave-safe mug (at least 12 oz capacity), whisk together 4 tablespoons all-purpose flour, 4 tablespoons granulated sugar, 2 tablespoons unsweetened cocoa powder, 1/8 teaspoon baking powder, and 1/8 teaspoon salt until well combined.
Add 3 tablespoons milk, 2 tablespoons vegetable oil, and 1/4 teaspoon vanilla extract to the dry ingredients in the mug.
Mix thoroughly with a fork or small whisk until the batter is smooth and free of lumps.
Place the mug in the microwave and cook on high power for 1 minute and 30 seconds.
Carefully remove the mug from the microwave (it will be hot) and let the cake cool for 2 minutes before eating.