Preheat oven to 350°. Line cookie sheets with parchment or silpat liners. In medium bowl, whisk together flour, cocoa, baking soda, and salt.
In a large bowl, cream together the butter and sugars until light and fluffy. Add eggs one at a time. Add vanilla and mix until well blended.
Add flour mixture and mix on low speed just until blended. Stir in chocolate chips and Sixlets. (I reserved about ⅓ of the Sixlets and pressed them into the top of the cookie immediately after they came out of the oven.)
Using a small cookie scoop, drop by rounded scoops on to cookie sheets. Bake for 8 minutes, until edges are set, but still soft in center. Let cookies cool on the cookie tray for five minutes. Remove cookies to a wire rack to cool.