In a microwavable mug (make sure that it has a 1-cup capacity, ~250 mL), stir together with a fork the flour, cocoa, brown sugar, baking powder and salt.
Measure the milk, oil and vanilla into the mug, and stir again until there are virtually no more lumps (be sure to scrape the bottom edges to make sure there are no flour pockets).
Top with chocolate chips.
Place the mug in the microwave, and cook on high for 90 seconds. The cake will puff up as it cooks. You can use a cake tester to double check if your mug cake is baked in the middle.
Carefully take the mug out of the microwave (it’s hot!) and let the cake sit for 5 minutes before diving in with spoon.