Preheat the oven to 350 degrees. Prepare two large rimmed baking sheets with parchment paper.
In a large bowl, whisk together the cocoa, flour, baking soda, and salt. Set aside.
In a mixing bowl, beat the butter and shortening until well blended.
Add the sugar and brown sugar to the butter mixture and beat until fluffy, about 6 minutes, scraping down the sides as needed. Then, mix in the eggs and vanilla extract.
To the wet ingredients, add the flour mixture and mix just until combined.
Fold in the chocolate chips and most of the Reese’s Pieces candies, reserving some of the candies for pressing into the top of each cookie.
Using a small cookie scoop or two spoons, drop balls of cookie dough onto the prepared cookie sheet about 2 inches apart. Press remaining candies into the tops of the cookies.
Bake for 8 to 9 minutes. Cool for 5 to 7 minutes on the baking sheet before transferring to wire racks.