In a stand mixer, using the paddle attachment, cream the butter, brown sugar, and sugar until smooth. Add the peanut butter and combine until smooth. Add the eggs and egg yolk and vanilla and mix until combined. Add the flour, salt, and baking soda and mix until just combined. Remove the paddle attachment and mix 1 cup of the Reese's Pieces candy into the dough by hand.
Scoop cookies into 1/4 cup cookie dough balls (size 16 cookie scoop), and refrigerate for one hour. While cookie dough is refrigerating, preheat oven to 350°F. Line a standard half sheet baking sheet with parchment paper and place 6 cookie dough balls on top in two rows. Press 5-6 Reese's Pieces into the top of the cookie dough balls.**
Bake for 10-12 minutes, until the edges and bottom of the cookies are golden brown. Repeat with remaining cookie dough until all the cookies are baked. Let cool for 10 minutes and serve.
Notes
*For best results use a name brand creamy peanut butter such as Jif or Skippy. Do not use natural peanut butter for this recipe.** For the most photogenic cookies, you can wait to place the reserved Reese's Pieces on top of the cookies 2-3 minutes before they are done baking. Simpy remove the cookie sheet from the oven, quickly place the Reese's Pieces on top of the cookies, and return to the oven to finish baking.