Cream together the butter, peanut butter, and the sugars until creamy. Add in the egg and mix until fully incorporated.
Whisk together the flour, baking soda and salt in a medium bowl. Slowly add to the peanut butter mixture, making sure to fully incorporate before adding the next batch of the flour mixture. Add in the milk and vanilla extract and mix until fully combined. Refrigerate for one hour to overnight.
Scoop about a 1-inch scoop of the peanut butter cookie dough and roll into a ball. Roll the cookie dough ball in the additional granulated sugar, place the peanut butter cookie dough ball about 2 inches apart onto the baking sheet. Bake 8 minutes and then gently press a chocolate kiss into the center of the cookie until the sides of the cookie begin to crack. Return to the oven for 2 more minutes. Remove from the oven and allow to cool.