Melt the butter in an 8oz (or larger) microwave safe mug (about 30 seconds).
Stir in the brown sugar, vanilla and salt.
Add the egg yolk and stir until smooth.
Stir in the flour and baking soda.
Stir in the chocolate chips.
Microwave for 45 seconds until the dough puffs and looks dry.
Carefully remove from microwave and enjoy!