In a microwave-safe mug (12-14 oz capacity), whisk together flour, sugar, baking powder, and salt with a fork until well combined. Create a well in the dry ingredients and add milk, oil, and vanilla extract all at once. Stir gently in figure-eight motions until just combined – don’t overmix. Batter should be smooth with no dry flour visible. Add optional mix-ins like chocolate chips if desired and gently fold in. Microwave on full power for 60-75 seconds, starting with 60 seconds and adding time in 10-second increments as needed. Cake is done when surface appears dry but still slightly springy to touch. Let cool for 1 minute before eating to allow steam to redistribute moisture.