In a medium bowl, whisk together the all-purpose flour, baking soda, and salt. Set aside.
Using a hand mixer or stand mixer fitted with paddle attachment, cream together the peanut butter, unsalted butter, granulated sugar, and light brown sugar on medium-high speed until well blended.
Add in the egg, milk, and vanilla extract. Scrape down the sides of the bowl as needed.
Reduce the mixer speed, gradually adding in the dry mixture until completely blended.
Place the cookie dough in a well sealed food container or tightly wrap the top of the mixing bowl with plastic wrap (or alternative) and chill the cookie dough for a minimum of 1 hour.
After dough has chilled, line 2 baking sheets with parchment paper or silicone mats, adjust the oven rack to the 2nd level mark (just above center), and preheat the oven to 375°F.
Using a spoon and your clean hands, shape the cookie dough into 1 inch balls, roll in granulated sugar, and place about 1 ½ - 2 inches apart on lined baking sheets.
Bake for 7-8 minutes, then remove from oven and place a milk chocolate kiss on each cookie, pressing down gently to create the crackly cookie edges..
Return to the oven and bake for 2 minutes longer.
Cool cookies on baking sheet for 5-10 minutes before transferring to a cooking rack to cool completely.
Store cookies in an airtight container with a slice of bread for up to a week.