Preheat the oven to 350℉/180℃. Line two baking trays with parchment paper and set aside. Melt the butter and allow it to cool as you prep the rest of the ingredients.
2/3 cups unsalted butter
In a bowl, add the flour, cocoa powder, baking soda, baking powder, cornstarch, and salt. Stir and set aside.
1 2/3 cups all-purpose flour, 1/3 cup unsweetened cocoa powder, 1 tbsp cornstarch, 1 tsp baking soda, 1/4 tsp baking powder, 1/4 tsp salt
In another large bowl, add the melted butter, brown sugar, and white sugar. Whisk until well combined (1-2 minutes).
2/3 cups dark brown sugar, 1/3 cups white sugar
Add in the egg, egg yolk, and vanilla. Whisk together for another minute or so until well combined, scraping the sides of the bowl as needed.
1 tbsp vanilla, 1 large egg, 1 large egg yolk
Add the dry ingredients in and mix together just until a soft dough forms, do not over-mix.
Mix in the chocolate chips.
3/4 cups semi-sweet chocolate chips
Scoop 6 cookies per tray, spacing them evenly apart from one another. I used 2 tbsp per cookie. Add extra chocolate chips on top if needed.
Bake for 13-15 minutes or until the edges are baked and the middles are slightly puffy and underbaked, they will continue to bake as they cool. Transfer to a cooling rack and enjoy!