Using a large mixing bowl, beat the butter and sugars for 4 minutes until pale and airy, achieving a smooth consistency free of grittiness.
Incorporate the peanut butter, egg, milk, and vanilla; continue beating for 2 minutes longer, frequently scraping down the bowl.
Fold in the flour, baking soda, and salt; stir until barely incorporated. If you have time, refrigerate for 30 minutes.
Heat oven to 375°F (190°C). Shape dough into 1-inch balls, coat in sugar, and bake for 8–10 minutes, until set but still tender and chewy.
As the cookies bake, remove wrappers from the Hershey Kisses. Immediately after baking, press one into each warm cookie. Let cool completely before transferring off the cookie sheet.