Preheat oven to 375℉ and prep cookie sheet with nonstick oil or liner.
In a large mixing bowl, sift together the flour, baking soda and salt.
Beat butter, peanut butter, sugars, egg, milk, and vanilla in an electric mixer then gradually add the flour until well combined.
Add some granulated sugar to a small bowl. Roll dough into 1 inch balls then dunk all sides in the sugar before placing on a cookie sheet 2 inches apart.
Bake until slightly puffed, ~6-8 minutes, then remove from the oven.
Gently press chocolate molds into each cookie, cracking slightly.
Bake in the oven for an additional 2 minutes then remove to cool completely. Store in an airtight container.