To the bowl of a stand mixer fitted with the paddle attachment (or large bowl and electric hand mixer), cream together the first 5 ingredients (through vanilla) on medium-high speed until light and fluffy, about 5 minutes.

Stop, scrape down the sides of the bowl, and add the flour, baking soda, optional salt, and mix on low speed until just incorporated, about 1 minute; don’t overmix.

Add the Reese’s Pieces, chocolate chips, and blend until just incorporated.

Using a medium 2-inch cookie scoop, form heaping two-tablespoon mounds (I made 27). Place mounds on a large plate, flatten mounds slightly with your palm, cover with plastic wrap, and refrigerate for at least 2 hours, or up to 5 days, before baking. Do not bake with warm dough because cookies will spread and bake thinner and flatter.

Preheat oven to 350°F, line a baking sheet with a Silpat or spray with cooking spray. Place mounds on baking sheet, spaced at least 2 inches apart (I bake 8 cookies per sheet).

Bake for 8 to 9 minutes, or until edges have set and tops are just beginning to set, even if slightly undercooked, pale and glossy in the center. Do not bake longer than 10 minutes because cookies 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.