In a microwave-safe mug, whisk together the flour, sugar, baking powder, and salt until well combined.
Add the milk, vegetable oil, fresh lemon juice, lemon zest, and vanilla extract to the dry ingredients in the mug.
Mix thoroughly with a fork or small whisk until a smooth batter forms with no lumps.
Place the mug in the microwave and cook on high power for 90 seconds.
Check the cake for doneness by inserting a toothpick into the center; it should come out clean or with a few moist crumbs.
If needed, microwave for an additional 10-15 seconds, but be careful not to overcook as the cake will continue to cook in the mug.
Allow the cake to cool for 2 minutes before eating directly from the mug or transferring to a plate.