Brown the butter

• Melt the butter in a saucepan over medium heat.
• Keep cooking until it foams, turns golden brown and smells nutty.
• Pour into a bowl and let cool slightly.

Mix the sugars and eggs

• Whisk the brown sugar and granulated sugar into the warm brown butter.
• Beat in the egg, egg yolk and vanilla extract until smooth and glossy.

Combine the dry ingredients

• Whisk the flour, baking soda, cornstarch and salt together in another bowl.
• Add the dry mixture to the wet ingredients and fold until just combined.

Add the chocolate

• Fold in the semi-sweet and dark chocolate chips until evenly distributed.

Chill the dough

• Cover the dough and refrigerate so the cookies hold their shape.

Bake the cookies

• Preheat the oven to 180 °C and line a baking tray.
• Scoop balls of dough onto the tray, spacing them apart.
• Bake until the edges are set and the centers are still soft.

Cool

• Let the cookies rest on the tray for a few minutes, then transfer to a rack to cool.