Step 1

First, start by melting the butter in the mug in the microwave. Add in the chocolate chips and stir until completely melted. Microwave the mixture as needed until the chocolate is completely melted.

Step 2

Add in the egg yolk, the brown sugar, and the vanilla if you're using some and stir until incorporated, Add in the flour and stir one more time just until the flour disappears. At this point you can add the chocolate chips and gently stir to get them incorporated to skip this step.

Step 3

Cook the brownie on high in the microwave for 45 seconds. Microwave temperatures vary so you can cook it for an additional 15 seconds if necessary.