Adjust oven rack to middle position and preheat oven to 375℉.
In a medium sized mixing bowl, whisk together flour, salt, and baking soda. Set aside.
In the bowl of stand mixer with paddle attached, beat together butter, peanut butter, granulated sugar, and brown sugar on medium speed until well combined and fluffy. Add egg, vanilla extract, and milk and continue mixing until smooth, scraping down sides as needed. Lower mixer speed and carefully add in flour mixture just until combined.
Use a small cookie scoop, or measure 1 tablespoon of cookie dough for each muffin tin. Bake for 8-10 minutes until light golden brown but still soft. Working quickly, immediately press chilled Reese's hearts into cookie dough. Hearts will get melty and then firm again as they cool. Once the chocolate is set again, remove cookie cups from mini tins. Alternatively, you can put the muffin tin in the freezer or fridge to speed the process.