Place butter in microwave-safe mug and microwave for 30 seconds until mostly melted.
Stir in brown sugar, white sugar, vanilla, and salt until well combined.
Add egg yolk and stir until mixture is smooth with no streaks.
Mix in flour until just combined - don't overmix. Fold in most chocolate chips.
Sprinkle remaining chocolate chips on top for decoration.
Microwave on high for 45-60 seconds until cookie puffs and looks set on top.
Let cool for 2-3 minutes before eating. Serve warm, optionally with ice cream.