How to Make It
Preheat oven to 375°F with racks in the upper and lower thirds of oven. Line 2 large rimmed baking sheets with parchment.
Beat butter, peanut butter, brown sugar, and 1/2 cup of the granulated sugar with a stand mixer fitted with paddle attachment on medium speed until fluffy, 2 to 3 minutes. Add egg, beating on medium speed until fully incorporated, about 30 seconds. Add milk and vanilla, and beat until combined, about 30 more seconds.
Stir together flour, baking soda, and salt in a medium bowl. With mixer running on low speed, gradually add flour mixture to peanut butter mixture, beating until well blended.
Shape dough into 1-tablespoon balls, and roll in remaining 1/2 cup sugar. Place balls 2 inches apart on prepared baking sheets. Bake in preheated oven until golden, 10 to 12 minutes, rotating baking sheets between racks and front to back halfway through bake time. Remove from oven, and immediately press a chocolate kiss into each cookie. Cool completely on baking sheets, about 20 minutes.