Preheat oven to 350 degrees.
In the bowl of a stand mixer, combine sugars, butter and peanut butter. Beat for 2 minutes or until smooth. Add vanilla, then mix. Add egg then egg yolk. Mix until just combined.
In a medium size mixing bowl, whisk together flour, salt, baking soda, baking powder. With mixing speed on low, gradually add dry ingredients- mix until just combined.
Remove bowl from stand and gently fold in Reese’s Peanut Butter Cup chunks. Form balls of dough using a standard size cookie scoop. Place on sheet and gently pat down with the palm of your hand (just flatten it slightly or use a fork and make a criss cross pattern in each).
Place in oven and bake for 12-14 minutes. Cookies will be slightly soft, but that’s ok. Let cookies cool on sheets for 5 minutes before transferring to a wire rack to cool completely. Serve with a big glass of cold milk!