Lightly coat the inside of an 8-ounce microwave-safe mug with nonstick cooking spray.

In a small microwave-safe bowl, combine creamy peanut butter and unsalted butter. Microwave for about 15–20 seconds until softened, then stir until smooth.

Sprinkle the light brown sugar into the warm peanut butter mixture. Stir until well combined.

Add in the lightly beaten egg yolk, milk, and vanilla extract; stir until smooth.

Gently fold in the all-purpose flour, baking powder, and a pinch of kosher salt.

Fold in the mini chocolate chips, ensuring they are evenly distributed.

Spoon the batter into the prepared mug and add a few extra chocolate chips on top if desired.

Microwave on high for 40 to 55 seconds, checking at the 40-second mark.

Let the mug cake cool for about 2 minutes before serving.

Enjoy warm, straight from the mug.