Select a microwave-safe mug that holds at least 12 ounces. Lightly spray the inside with cooking spray or brush with a thin layer of butter to prevent sticking.
Add the flour, sugar, baking powder, and salt directly to the mug. Whisk together with a fork until evenly combined with no dry pockets visible.
Pour in the oil and vanilla extract, stirring until incorporated. Add the milk and stir gently for 10-15 strokes until just combined — small lumps are okay and help maintain a fluffy texture.
Tap the mug gently on the counter 2-3 times to release any large air bubbles trapped in the batter.
Microwave on high power for 60 seconds. The cake should rise significantly and the top should look just set with no wet batter visible. If your microwave is less than 1000 watts, add 10-second bursts as needed.
Let the cake cool in the mug for 30 seconds to allow the texture to set properly. Top with whipped cream, ice cream, or a dusting of powdered sugar if desired, then enjoy straight from the mug with a spoon.
