Preheat your oven to 350°F (175°C). In a medium mixing bowl, sift together flour, cocoa powder, cornstarch, baking soda, and salt. Set aside.

In a microwave-safe bowl, combine semi-sweet chocolate and heat in 20-second intervals until melted. Let cool slightly.

In a large mixing bowl, beat unsalted butter, brown sugar, and white granulated sugar until light and fluffy. Add vanilla extract and cooled melted chocolate, and mix until smooth.

Incorporate the large egg and egg yolk, beating until fluffy and pale.

Gradually add the sifted dry ingredients to the chocolate mixture, mixing just until combined. Fold in white and semi-sweet chocolate chips.

Cover the dough and refrigerate for 30 minutes.

Scoop large dough balls onto baking sheets, spacing them 2 inches apart. Bake for 13-15 minutes until edges are set but centers are soft.

Let cookies rest on the baking sheet for 5 minutes, then transfer to a cooling rack.