Mix butter and sugars

• Whisk the melted butter with the granulated sugar and light brown sugar until the mixture looks like wet sand.

Add egg and vanilla

• Add the egg and vanilla extract and whisk until fully combined.

Add dry ingredients

• Sift in the flour, baking soda and salt.
• Fold until almost combined, then fold in the dark and milk chocolate chips.

Scoop and chill the dough

• Scoop the dough into 9–11 portions.
• Chill in the fridge for at least 1 hour or overnight.

Preheat the oven

• Preheat the oven to 180 °C (350 °F).

Shape and bake

• Roll the chilled dough into balls and place them on a lined tray, 2 inches apart.
• Bake for 9–12 min until the edges are set; underbake slightly for chewy cookies.

Cool the cookies

• Cool on the tray for 5–10 min, then transfer to a rack to cool completely.