Melt the butter in the mug

• Place the butter in a mug and melt it in the microwave.

Mix the wet ingredients

• Add the brown sugar, granulated sugar, egg yolk and vanilla extract.
• Stir until smooth.

Add the dry ingredients

• Stir in the flour, salt and chocolate chips until just combined.

Microwave the cookie

• Microwave until the cookie is set on top and still soft in the middle.

Rest and serve

• Let the cookie cool slightly in the mug before eating.