Sift together the all-purpose flour, unsweetened cocoa powder, cornstarch, baking soda, and salt into a bowl. Set aside and melt the semi-sweet chocolate in a microwave, stirring frequently.

In a large mixing bowl, cream the unsalted butter with both sugars on high speed for 2 minutes. Incorporate the vanilla extract and melted chocolate, mixing for another 2 minutes.

Add the egg and egg yolk, beating on high speed for 1 minute until fluffy.

Gradually mix the dry ingredients into the wet mixture until just combined. Fold in the white and semi-sweet chocolate chips.

Chill the dough in the refrigerator for 30 minutes. Preheat the oven to 350°F (175°C) and line cookie sheets with parchment paper.

Use a cookie scoop to form dough balls on the sheets, leaving 3 inches between each. Bake for 13-15 minutes until edges are firm but centers are soft. Let cool on the baking sheet for 5 minutes before transferring to a wire rack.