In a medium bowl, stir together the flour, baking soda, and salt. Set aside.
In a large bowl, beat the butter and peanut butter with an electric mixer until well combined. Add granulated sugar and brown sugar; beat until fluffy. Add the egg, milk, and vanilla; beat well. Gradually beat the flour mixture into the peanut butter mixture.
Shape dough into 1-inch balls. Roll in granulated sugar, if desired. Place on ungreased cookie sheet.
Bake 8-10 minutes or until lightly browned. Immediately press a Hershey Kiss into the center of each cookie. Allow the cookies to cool on the pan for 3 minutes before transferring to a wire rack to cool completely.
Notes
Be sure to use a cookie scoop or tablespoon measure to ensure all of your cookies are uniform in size.
It's best to keep the cookie dough right about 1-inch so they bake properly.
For pretty holiday cookies, roll your cookie dough balls in colored sugar or sprinkles before baking.
To ensure your kisses don't over melt, you can refrigerate them before adding them to the tops of the cookies.