In a light saucepan, melt the butter. I prefer to brown my butter - in order to do this, cube the butter, and melt over a medium heat, stirring constantly. The butter should begin to foam and bubble. Keep cooking it until the butter is golden brown, with a deep buttery/nutty scent. Pour this into a heat-proof bowl straight away.

Once the butter has cooled to room temperatures, add in both sugars, and whisk until combined.

Beat in the egg, egg yolk, and vanilla.

Fold in the flour, bicarbonate of soda, and salt until just combined.

Fold through the chocolate chips.

Cover the bowl with cling film, and refrigerate for at least half an hour, up to 2 hours, before baking.

Preheat the oven to 180°C. In the mean time, scoop the dough with an ice cream scoop and place the balls on a baking tray, leaving a gap between them.

Bake for 10 - 12 minutes, or until golden brown. The cookies should be soft - they will firm up as they cool.