Mix in the mug: Choose a large, microwave-safe mug, at least 350 ml, because the cake rises a lot. Put the flour, cocoa, sugar, baking powder, and salt in the mug and stir with a fork until combined and free of lumps.
Add the wet ingredients: Pour in the milk, oil, and vanilla and stir until you have a smooth batter with no dry pockets. Stir in the chocolate chips, if using. Wipe any batter off the rim of the mug.
Microwave in bursts: Microwave on high for 60 seconds, then check: the top should be set but still slightly soft in the very centre. If it still looks wet, add 10 seconds at a time. Most microwaves need 60 to 90 seconds in total. Do not cook until it is firm all over, or it will be overdone by the time you eat it.
Rest and serve: Rest for 1 to 2 minutes. The cake is very hot and the centre finishes setting as it stands. Top with a scoop of vanilla ice cream and eat from the mug with a spoon while it is warm.