Begin by melting butter in the microwave. This usually takes about 30-45 seconds. Stir in brown sugar and granulated sugar, mix until combined.
Add in egg yolk, vanilla extract, peanut butter and milk and mix until combined.
Next fold in the flour and then the chocolate chips.
Place cookie in the microwave and cook for 45 seconds to a minute on full power. If cookie still looks too runny or undercooked let it go a bit longer. Keep in mind the cookie will continue to cook as it cools. Take cookie out of the microwave and let sit for 2 minutes then serve!