Place the butter into a large ramekin or mug. Melt for 30 seconds (or until melted).
Stir through the caster sugar, brown sugar, vanilla extract and a pinch of salt.
Stir through the egg yolk until well combined.
Mix through the plain flour and then the chocolate chips.
Cook for 40 seconds or until just set on top (do NOT overcook).
Serve immediately while still warm.
Notes
RECIPE NOTES & TIPS
All microwaves are different - start by microwaving the cookie for 40 seconds. If it's still gooey on top, cook for 10-20 seconds longer (in 5 second increments) until just set.
Careful not to overcook - you want this cookie mug to still be a little gooey inside.
Use a large mug - or split into two mugs, so that the mug cake doesn't overflow when being microwaved.