Combine the first four ingredients in a bowl, and whisk so there are no clumps.

Next add in the milk, oil, and vanilla extract. Whisk well.

Then add in the sourdough starter. Whisk this in quickly. The mixture will become airy and start to rise.

Fold in the chocolate chips.

Divide the batter evenly between two mugs and place in the microwave together. Cook for 2 minutes and 30 seconds.

Then let the mug cake rest for 1 minute before eating.