Preheat your oven to 350°F and line baking sheets with parchment paper or silicone mats.
In a medium bowl, whisk together the flour, cocoa powder, baking soda, and salt. Set aside.
Using an electric mixer, beat the butter, granulated sugar, and brown sugar together until fluffy, about 2-3 minutes.
Add the egg, egg yolk, and vanilla extract, beating until well combined and smooth.
Gradually mix in the dry ingredients on low speed until just combined—don't overmix!
Fold in the chocolate chips until evenly distributed throughout the dough.
Scoop heaping tablespoons of dough and roll into balls, placing them 2 inches apart on the prepared baking sheets.
Bake for 9-11 minutes—the cookies should look slightly underdone in the center with set edges.
Allow cookies to cool on the baking sheet for 10 minutes before transferring to a cooling rack. They'll continue to set as they cool.