Line baking sheets with parchment paper or silicone mats.
In a large bowl whisk together the flour, baking soda and salt; set aside.
In a mixer cream together the butter, peanut butter and sugars until lightened and fluffy, about 2 minutes.
Add the egg, milk and vanilla and mix until incorporated. Stir in the flour mixture and mix until a dough forms. Scrape sides of bowl to make sure everything is mixed evenly.
Wrap and chill the dough about 30 minutes so it's easier to work with. Heat oven to 350ºF.
Scoop the dough into 2-tablespoon-size balls (I used a small cookie scoop). Roll the balls in extra sugar and place about 1 inch apart on the baking sheets. Bake until just done, but not browning, 9 to 10 minutes. Immediately place a Hershey Kiss in the center of each cookie, gently pressing down. Let cool for about 15 minutes before moving to a cooling rack to cool completely. Can be stored airtight for 3 to 4 days.