In a microwave-safe mug (around 12 oz), combine the flour, sugar, cocoa powder, baking powder, and salt. Stir until well mixed.
Add in the milk, vegetable oil, and vanilla extract. Mix thoroughly until the batter is smooth and no lumps remain.
Gently fold in the semi-sweet chocolate chips, white chocolate chips, and dark chocolate chips until evenly distributed in the batter.
Microwave the mug on high for 1 minute and 30 seconds. If the cake is still wet in the center, continue to microwave in 15-second increments until cooked through but moist.
Allow the mug cake to cool for a minute before diving in.
Notes
Serve warm with whipped cream and cocoa powder or chocolate shavings.