In a large microwave-safe mug, whisk together the flour, cocoa powder, sugar, baking powder, and salt with a fork.
Add the milk, oil, and vanilla, then stir until completely smooth, scraping the bottom and corners of the mug so no dry pockets remain.
Sprinkle the chocolate chips over the top and press them in slightly - they melt into a gooey center.
Microwave on high for 70-90 seconds, until the cake has risen and the top looks set. Do not overcook, as it keeps the cake fudgy.
Let the mug rest for 2 minutes before digging in - it is delicious on its own or topped with a scoop of vanilla ice cream.
