Very lightly coat the inside of a 6- or 8-ounce mug or ramekin with cooking spray, butter or coconut oil.

To the mug or ramekin, add the melted butter, egg yolk, granulated sugar, brown sugar, vanilla and salt. Mix until well combined.

Add the flour and chocolate chips. Stir until just combined. If the batter is dry and crumbly, add a splash of milk.

For a 1,000 watt microwave, cook for one minute at 80% power. For a 1,200 or 1,500 watt microwave, cook for 45 seconds at 75% power. The key is to NOT OVERCOOK the mug cookie.

Serve warm with ice cream, if desired.