Whisk together the dry ingredients - all-purpose flour, cocoa powder, baking powder, granulated sugar and salt in one medium bowl.
Once it's mixed, whisk in the milk and vegetable oil until all ingredients are combined, and the batter is smooth.
Pour batter into a microwave-safe mug. I used 12-ounce mug. Don't fill a mug more than half full. You have to give enough space for the cake to rise.
Microwave mug cake for 70 seconds on high. It should be firm on the top and a cake tester inserted in the center of the cake shall come out clean. If it's not done, keep cooking it in 15-second bursts. All microwaves are different, so suggest to follow this step while microwaving the cake.
Once done, remove the cake from the microwave. Enjoy!
Notes
To make the mug cake, any microwaveable cup will work. It can be glass or ceramic.
Don't fill the cake batter, more than half of the cup.