Preheat oven to 350°F (175°C).  Prepare a baking sheet with parchment paper.

In the bowl of a large stand mixer, use the paddle attachment to beat butter, granulated sugar, and brown sugar together on medium-high speed until light and fluffy (about 2 minutes). Add in eggs and vanilla, and beat on medium-speed until combined.

In a separate bowl, whisk together the flour, cocoa, baking soda, and salt until combined.  Gradually add the dry flour mixture into the butter mixture and beat on medium-low speed until combined.  Fold in the chocolate chips until combined.  Cover and chill the dough in the refrigerator for at least 30 minutes.  (Or you can wait until after the next step, and chill the dough once it has been formed into balls and placed on the cookie sheet, which only takes 10-15 minutes.)

Roll the dough into 1.5-tablespoon balls, and place on the parchment-covered baking sheet so that the dough balls are at least 3 inches apart.  Bake for 8 to 10 minutes, or until the cookies are cooked through.  Remove and let cool for 5 minutes, then transfer the cookies to wire racks to cool completely.