Preheat oven to 350 degrees F and line 3 baking sheets with parchment paper (optional).
Cream together butter and sugar with an electric mixer until smooth. Add vanilla and eggs and combine.
Add flour, baking soda, cocoa, and salt and beat until a dough forms (it should hold together easily but be dry enough that you can roll into balls without coating your hands, but not crumbly). Stir in chocolate chips and Reese's Pieces.
Roll into 1" balls and place about 2" apart on prepared baking sheets (you can make larger cookies, but you'll need to be fewer to a pan and bake a little longer). Bake for 8-9 minutes until centers are just set or only a tiny bit glossy in the center -- they will set as they cool.