Whisk flour, baking soda, and salt; set aside.
In a large bowl, using an electric mixer, beat the butter and peanut butter until creamy, 1 to 2 minutes.
Mix in brown sugar and 1/4 cup granulated sugar until smooth.
Beat in egg, then vanilla and milk.
Add dry ingredients; mix just until a soft dough forms.
Scoop tablespoon-size dough onto a sheet and chill 15–20 minutes.
Preheat oven to 350°F. Line baking sheets with parchment.
Roll dough into balls, coat in remaining sugar, and place 2 inches apart.
Bake 8–10 minutes, until tops crack and edges look set (no browning).
Cool 5 minutes, then press a Hershey’s Kiss into each cookie.
Let sit 5–10 minutes more, then transfer to a rack to cool completely.