Zero out a scale and place a bowl on top (if you’re weighing the ingredients). Combine the flour, sugar, baking powder, and salt in the bowl. Whisk so there are no clumps.
Next, add in the milk, oil, peanut butter, and vanilla extract. Whisk well.
Then add in the sourdough starter. Whisk this in quickly. The mixture will become airy and start to rise.
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. Enjoy warm, and serve with ice cream on top if you’re feeling fancy!