Step 1

Place the butter in a mug (an 8-ounce mug is the perfect size, but larger will work too) and melt in the microwave for about 30 seconds. Using a small spoon, stir in the brown sugar, vanilla, and salt (don't worry if it looks a little separated). Add the egg yolk and stir until smooth. Stir in the flour and baking soda. Stir in two-thirds of the chocolate chips, and then sprinkle the remaining chips on the top. Microwave on high heat for about 45 seconds, until the dough puffs and looks dry. Remove from the microwave and let cool for a few minutes before eating. (The cookie should be eaten warm; it dries out once cool.)

Step 2

Pro Tip: For clean edges, use a paper towel to wipe off any batter from the inside of the cup (up to the level of the batter) before baking.