•  1 cup unsalted butter melted and then cooled for at least 5 minutes (226g) • 1 cup light brown sugar tightly packed (200g) • ¾ cup sugar (150g) • 2 large eggs + 1 egg yolk room temperature • 1 ½ teaspoons vanilla extract • 2 ¼ cups all-purpose flour (285g) • 2 teaspoons cornstarch (cornflour in the UK) • ½ teaspoon baking powder • 1 teaspoon salt • 2 cups semisweet chocolate chips (340g)

Combine melted (cooled)  butter and sugars in a large bowl, stir well.

Add eggs, egg yolk, and  vanilla extract and stir until well-combined.

In a separate, medium-sized bowl, whisk together flour, cornstarch, baking powder,  and salt.

Gradually stir dry ingredients into wet until completely combined. 

Stir in chocolate chips.

Preheat oven to 350F (175C) and prepare a 9x13 pan by lining with parchment paper (or lightly grease and flour).

Spread batter evenly into prepared pan. Bake on 350F (175C) for 25-30 minutes or until edges just begin to turn golden brown.

Allow to cool before cutting and serving (don't cool too long, though -- these are amazing when served still slightly warm!)