Using a mixer, cream together the first 5 ingredients (through vanilla) on medium-high speed until light and fluffy, about 5 minutes, scrapping the sides of the bowl, as needed.
Add the flour, baking soda, salt, and mix on low speed until just incorporated, about 1 minute; do not over mix.
Using a spatula or wooden spoon, add the Reese's Pieces, chocolate chips, and blend until just incorporated.
Once the dough is chilled, pre-heat oven to 350 degrees. Note: if you chilled your dough overnight or longer, you will need to remove the dough from the fridge while the oven preheats. See note above.
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.