Prepare your mug by lightly greasing the inside with butter or cooking spray to ensure easy cleanup (though this is optional).
Combine the dry ingredients by adding the flour, sugar, baking powder, and salt to your mug. Stir with a fork until well mixed.
Add the wet ingredients – pour in the milk, oil or melted butter, and vanilla extract. Mix thoroughly until no lumps remain, making sure to scrape the bottom of the mug where flour often hides.
Fold in the chocolate chips gently with your fork, distributing them evenly throughout the batter.
Sprinkle the additional tablespoon of chocolate chips on top if using (this creates a delicious melty top layer).
Microwave on high for 60-90 seconds. Start with 60 seconds for a gooier center, and add additional 10-second intervals if needed. The cake is done when it rises and the top looks set but still slightly moist.
Let stand for 2 minutes before eating – this allows the cake to finish cooking and prevents burning your mouth on molten chocolate chips.