One of our go-to Christmas cookies! Chocolate kisses are perched atop textbook peanut butter cookies in these sweet-and-salty treats.
Sift flour, baking soda, and salt into a bowl. In the bowl of a mixer fitted with the paddle attachment, beat together butter, peanut butter, and both sugars on medium-high speed until light and fluffy, 2 to 3 minutes. Beat in egg and vanilla. Reduce speed to low; alternate adding flour mixture and milk, beating until well combined. Cover and refrigerate 30 minutes. Shape dough into 1-inch balls (1 level tablespoon each) and transfer to parchment-lined baking sheets, spaced 1 inch apart. Freeze 30 minutes.
Preheat oven to 375 degrees. Bake, rotating pans halfway through, until puffed, lightly cracked on top, and just set around edges, 12 to 15 minutes. Remove from oven and press a chocolate kiss into center of each cookie. Let cool on sheets 5 minutes, then transfer cookies to wire racks and let cool completely.