In a large bowl cream together butter and sugar until smooth and pale colored. Add eggs and vanilla and beat until mix until incorporated.
Stir together flour, cocoa powder, baking soda, and salt; add to butter mixture. Stir in mint chocolate pieces, reserving about 1/4 cup for pressing into the tops of the rolled cookie dough.
Spoon and roll cookie dough into desired size (I like mine bigger--about 2-3 tablespoons) Place spread apart on a baking sheet lined with parchment paper or silpat liner. Bake for 10-12 minutes or just until set. (Cookies wont be shiny on top, but may appear slightly soft and will set up after removing them from the oven to cool for a few minutes.)
Make ahead Instructions: Make the dough up to 2 days ahead of time and store it in the refrigerator. Freezing Instructions: The cookie dough balls may be frozen for up to 3 months. Bake from frozen, adding a few extra minutes to the cook time. To freeze the baked cookies, allow them to cool completely and store in a freezer-safe bag for up to 3 months.