In the bowl of a stand mixer or large mixing bowl, with the paddle attachment, on low speed to blend well the unsalted butter, creamy peanut butter, white sugar, and brown sugar. The mixture will be smooth.
To the stand mixer bowl, using low speed, mix in the egg, milk, and vanilla extract until well blended.
Gently stir a little at a time, the flour, baking soda, and salt, to just blend the ingredients.
Place the bowl in the refrigerator for about 30 minutes to chill dough.
Preheat oven to 375°F/191°C. Place a Silpat on a baking tray or spray the baking sheet with non-stick baking spray. Set aside.
Remove dough from the fridge and roll into 1 inch balls.
Roll each ball into the white sugar. Place on the prepared baking tray.
Bake for 8-10 minutes in the preheated oven. The cookies will be soft and light brown.
Quickly press in a chocolate kiss candy into the center of each cookie. The cookie will crack slightly.
Place them on a cooling rack to harden slightly as they cool.
Continue until all the dough is used to make cookies.