In a microwave-safe mug that holds about 12 oz, combine the all-purpose flour, granulated sugar, unsweetened cocoa powder, baking powder, and salt. Use a fork or small whisk to stir the ingredients together until they are well blended and free of lumps.
Pour in the milk, vegetable oil, and pure vanilla extract. Stir vigorously until the batter is smooth and uniform, ensuring that there are no dry bits of flour remaining.
Gently fold in the semi-sweet chocolate chips, white chocolate chips, and dark chocolate chips. Ensure that the chocolate pieces are evenly distributed throughout the batter for a deliciously gooey texture.
Place the mug in the microwave and cook on high for 1 minute and 30 seconds. After this time, check the cake's center; if it appears wet, microwave in additional 15-second increments until the cake is cooked through but still moist to the touch. Be cautious not to overcook.
Once cooked, carefully remove the mug from the microwave. Allow the mug cake to cool for about 1 minute before indulging.
Notes
For an elegant finish, serve warm topped with whipped cream and a dusting of cocoa powder.