Add butter to a microwave-safe 12 oz mug and microwave until melted, about 20 to 30 seconds. Swirl to coat the sides.
Whisk in sugar, milk, and vanilla until well combined.
Add flour, baking powder, and salt. Stir just until a smooth batter forms; do not overmix.
Fold in chocolate chips, reserving a few to sprinkle on top.
Microwave on high for 60 to 90 seconds until the cake rises and the top looks set but still slightly glossy in the center. Do not overcook.
Let stand 1 minute to finish setting. Enjoy warm straight from the mug.
Notes
Microwaves vary—start with 60 seconds and add 10-second bursts until done. For extra indulgence, top with a scoop of vanilla ice cream or a drizzle of chocolate sauce.