Pre-heat oven to 350 degrees and lightly grease a 9×13 baking dish.
In the bowl of a stand mixer or in a large bowl with a hand mixer, cream together butter and sugar until light and fluffy.
Beat peanut butter into the mixture until smooth. Add eggs one at a time, mixing well after each addition. Beat in vanilla.
In a separate large bowl, whisk together flour, cocoa powder, and salt.
Add the dry mixture to the wet ingredients and mix until just combined. Fold peanut butter cups into batter.
Pour the mixture into prepared baking dish and sprinkle Reese's Pieces over the top.
Bake at 350 degrees for about 35 minutes, until a toothpick comes out clean. Set pan on a wire rack and let cool for about 10 minutes before slicing.