Preheat oven to 350 degrees Fahrenheit and prepare cookie sheets by lining them with nonstick baking mats or lightly spraying with nonstick baking spray.
In the bowl of an electric mixer, cream together butter, brown sugar, white sugar, eggs and vanilla. Add melted semi-sweet chocolate chips and mix until incorporated.
In a separate bowl, stir together flour, baking cocoa, baking soda, and salt. Add it to the wet mixture and mix until incorporated. Add in 1 c. mint chocolate baking pieces and 1 c. semi-sweet chocolate chips and quickly mix to incorporate.
Scoop dough into rounded balls with a 1 1/2" cookie scoop. Mine were heaping. Place at least 2" apart on the cookie sheet. Use the flat side of a cup to press the dough balls down to flatten a little.
Bake for 8 minutes. Let cool on cookie sheets for about 5 minutes before carefully removing to a cooling rack to cool completely. Store in an air-tight container once cool.