Preheat Oven to 350° and line 2-3 large baking sheets with parchment or a silicone non stick baking mat.
Combine the flour, cocoa powder, baking soda and salt in a small bowl. Set aside.
In the bowl of a stand mixer, use the paddle attachment to beat the butter until creamy, about 1 minute.
Add sugar to mixer bowl with creamed butter, and beat on medium speed until light and fluffy, about 2-3 minutes.
Add egg, egg yolk and vanilla to mixer bowl and beat on low until just combined.
Add your dry ingredients and your dark chocolate chips to mixer bowl and beat on low until just combined.
Form cookie dough balls with about 2 TBS of dough, and place 8 cookies on a sheet pan.
Bake for 9-11 minutes. The inside should still be a little gooey, while the exterior should be crackly.
Place sheet pan on a wire rack to cool. Once cookies have cooled about 5-10 minutes, transfer cookies to wire rack and let them cool completely. Enjoy!