Using a mixer, cream together butter and sugars on medium-high speed until light and fluffy, about 5 minutes, scrapping the sides of the bowl, as needed. In a separate bowl, whisk flour, cocoa powder, baking soda. Add to wet ingredients and mix on low just until combined. Add the Reese's Pieces and peanut butter chips, and blend until just incorporated.
Using a medium cookie scoop, scoop heaping spoonfuls of dough and place on a large serving platter or plate. Roll the mounds into smooth balls and slightly flatten with your palm. Cover with plastic wrap, and refrigerate for at least 3 hours, or up to 3 days, before baking. Do not bake with warm dough because cookies will spread and bake thinner and flatter.
Once the dough is chilled, pre-heat oven to 350 degrees. Place dough on cookie sheets lined with parchment paper, about 2 inches apart from one another. Bake for 8 to 9 minutes, rotating cookies halfway through. Do not bake longer than 10 minutes, as they will firm up as they cool.
Allow cookies to cool on the baking sheet for 5 minutes before removing and transferring to a rack to finish cooling.