Preheat oven to 375 degrees. Line two baking sheets with parchment paper and set aside.
Beat softened butter and peanut butter in a large bowl until well blended. Measure and add in 1/3 cup granulated sugar and brown sugar. Stir until light and fluffy. Add egg, milk, and vanilla; stir well.
Stir flour, baking soda, and salt into the peanut butter mixture until just incorporated. Scrape the sides of the bowl and mix again briefly.
Scoop into cookies using a 1 1/2 tablespoon scoop. Roll in between your palms, and then roll in sugar. Place on baking sheets.
Bake for 8-10 minutes or until very lightly golden in color. Remove from oven and immediately press a Hershey's Kiss into each cookie. Cookies will crack around the edges with you press the chocolate in…totally normal! Cool for 5 minutes on the hot baking sheet and then remove to cooling racks.
Store cooled cookies in airtight containers for up to three days.