Preheat oven to 350 degrees F.
In the bowl of a standing mixer, add the room temperature butter, peanut butter, sugar and brown sugar. Using the paddle attachment, cream until light and fluffy, or about 2 minutes.
Add in the eggs, then mix on low to combine. Then add in the vanilla, salt and milk and mix on low to combine.
In a separate bowl, whisk together the flour and baking soda, then add this to the butter mixture and mix on low, until almost combined. Scrape down the sides of the bowl as needed to be sure there are no lumps of butter in your dough, then add in your Reese's Pieces and stir in gently by hand until they are evenly incorporated.
Once mixed, scoop and roll the cookie dough into 1 ¼" balls. Place on a baking sheet lined with parchment paper, or a Silpat, and press them down gently to flatten slightly.
Bake for 7-8 minutes, or until the bottoms and edges are very lightly browned.
Remove from the oven and sprinkle a few more Reese's Pieces on the tops of each cookie if you would like. Let them cool for 2 minutes on the baking sheet before transferring to a wire rack. Enjoy!