Melt the butter

• Melt the butter and let it cool slightly.

Mix the wet ingredients

• Whisk the melted butter with the brown sugar and granulated sugar until smooth.
• Add the egg, egg yolk and vanilla extract and whisk until glossy.

Add the dry ingredients

• Whisk the flour, baking soda and salt together.
• Fold the dry mixture into the wet mixture with a spatula until just combined.

Fold in the chocolate chips

• Fold the chocolate chips through the dough.

Chill the dough

• Cover the dough and chill it so the cookies bake up thin and chewy.

Preheat the oven

• Preheat the oven to 180 °C and line a baking tray.

Shape the cookies

• Roll the dough into balls and space them well apart on the baking tray.

Bake

• Bake until the edges are set and the centres still look soft.

Cool

• Leave the cookies on the tray to firm up, then transfer them to a cooling rack.