In large mixing bowl, use an electric hand mixer to beat butter and sugars until well combined and creamy. Add eggs, one at a time, beating after each addition. Add vanilla, beating for another minute just until incorporated.
In a separate bowl, whisk together the flour, cocoa, baking soda, and salt. Add the dry mixture to the butter mixture and beat until well combined. Use a spoon to mix in the chocolate chips. Cover and refrigerate dough for at least an hour or more.
Remove chilled dough from refrigerator and drop by rounded spoonfuls onto parchment lined or ungreased cookie sheets.
Bake for 8 to 10 minutes in the preheated oven, or just until set. Cookies will seem slightly soft but don't be tempted to bake longer. Allow to cool on cookie sheet for 3 to 4 minutes then transfer to wire racks to cool completely.
Prep time includes one hour of chilling time for the dough.