Choose a microwave-safe mug/ramekin/small bowl. I used an 8-ounce ramekin (4 inches in diameter).
Place the butter in the mug and melt in the microwave; do this in 15 second intervals at about 70% power. When you have small bits of butter remaining, take it out of the microwave and gently swirl, or stir, the butter until the bits have melted.
To the melted butter, add sugars, vanilla and salt. Stir well.
Add the egg yolk and stir well to combine.
Add the flour, and stir until combined. The consistency of the dough should be like wet cookie dough; if it's too runny, add a touch more flour until the consistency is firmer, or if it's too stiff, add a splash of milk.
Add the chocolate chips and stir to evenly distribute throughout the dough. Flatten the surface of the dough with the back of the spoon.
Microwave at about 100% power for 30-40 seconds. Continue in 10 second intervals until the cookie is done (mine took 60 seconds). If you overcook the cookie, it will be become dry and hard. Always err on the side of less cooking time.