Preheat the oven to 350F. Line 3 baking sheets with parchment paper or a silicone baking mat.
Mix together the flour, cocoa powder, cornstarch, baking soda, and salt in a medium bowl. Set aside.
Add the butter and both sugars to a mixer and beat on medium speed until light and fluffy, about 3 - 4 minutes.
Add the eggs and vanilla to the mixer and beat for another minute to combine.
Beat in the flour mixture until just combined. Be careful to not over mix.
Fold in the chocolate and walnuts.
Portion out your dough into 10 equal sized balls of cookie and separate at least 2 -inches apart.
Bake for 12 - 15 minutes, or until the edges are just set. Remove from the oven and allow to cool on the baking sheet before transferring to a wire rack to cool completely.
Notes
Make sure to measure your flour properly by giving it a quick fluff, and then spooning it into your measuring cup and sweeping off the excess with the flat edge of a utensil. Make sure your butter and eggs are at room temperature. You can learn how to quickly soften butter here and you can bring eggs to room temperature quickly by placing them in warm water for five minutes. Use natural cocoa powder instead of Dutch processed cocoa powder.