Partially melt 2 tablespoons butter in microwave at 70% power for 20 seconds.
Add 2 tablespoons sugar, 1 1/2 tablespoon brown sugar, and 1/8 teaspoon vanilla to melted butter.
Stir.
Add 1 egg yolk and stir.
Add 3 tablespoon flour and a pinch of salt. Stir.
Add 2 tablespoons chocolate chips. We used semi-sweet mini-chocolate chips, but you can use your favorite kind.
Stir.
Grease coffee mug. We recommend using a stick of butter for this.
Add mixture to mug and spread evenly throughout.
Microwave at 50% power for 60 seconds. Rotate the mug and microwave at 70% power for 30 seconds. Finally, rotate again and microwave at 70% for 30 seconds. Allow 10 minutes to cool.
Add your favorite toppings. Chopped pecans, ice cream, whipped topping, chocolate sauce, and/or caramel sauce.