Using an electric mixer, cream together butter, granulated sugar, brown sugar, eggs and vanilla on medium speed until light and fluffy.
In a separate bowl, whisk together flour, cocoa powder, baking soda and salt.
With the mixer on low, gradually add the dry ingredients to the butter mixture. Beat until well blended, stopping to scrape the sides and bottom of the bowl as necessary to make sure that everything is evenly incorporated.
Stir in chocolate chips. Cover dough and chill for at least 2 hours.
Preheat oven to 350° F. Line baking sheets with parchment paper or silicone mats.
If the dough is too firm to scoop after chilling, allow the bowl to sit on the counter at room temperature for 10-15 minutes so that it softens slightly. Drop the dough by rounded tablespoons two-inches apart on the prepared baking sheets.
Bake for 8-10 minutes, or just until the edges are set. The centers will still be soft.
Cool on the baking sheets for about 3-5 minutes, then transfer to wire racks to cool completely.