Preheat your oven to 375°F and prepare two large baking sheets by lining them with parchment paper. In a small bowl, whisk together the flour, baking soda, and kosher salt until evenly combined.

In a separate large mixing bowl, use a hand mixer to cream together the butter and peanut butter until the mixture is smooth. Gradually add the brown sugar and granulated sugar, beating until the mixture is light and fluffy. Beat in the egg, followed by the milk and vanilla extract, mixing until all ingredients are fully incorporated.

Slowly add the flour mixture to the wet ingredients, stirring on a low speed until the dough comes together.

Shape the cookie dough into balls using approximately 2 tablespoons of dough per ball. Roll each ball in the additional granulated sugar, ensuring an even coating. Arrange the dough balls on the prepared baking sheets, making sure to space them at least 2 inches apart to allow for spreading.

Bake in the preheated oven for 8 to 10 minutes, or until the cookies have puffed up and the surface no longer appears raw. Note that these cookies will not brown as they bake.

As soon as the cookies come out of the oven, press a Hershey Kiss into the center of each cookie. Let them cool slightly before serving.