Chocolate Chip Cookie in a Cup

Butter

Granulated White Sugar

firmly packed Dark Brown Sugar

Drops of Vanilla Extract

Small Pinch of Kosher Salt

Egg Yolk (discard the egg white or save for different recipe)

Scant 1/4 of All Purpose Flour (slightly less than 1/4 of a cup)

heaping tablespoons of Semi Sweet Chocolate Chips plus more for topping