Line a cookie sheet with parchment paper. Slightly grease the the paper with cooking spray or coconut oil. Set it aside.
In a mixing bowl, beat egg and sugar with a fork or hand mixer until combined and slightly frothy.
Next, add in the peanut butter and beat on low with an electric hand mixer until a crumbly dough forms. The dough should come together into a dough ball when you press it together.
Form 12 1-inch cookie dough balls. Place them onto the prepared cookie sheet leaving about an inch between each. Slightly press down the ball with the palm of your hand to form a thick patty about 3/4 inch in thickness, fixing any large cracks that form with your fingers. At this point, you may wish to make an indent in the centre of each cookie with the bottom of the Hershey's Kiss to make it easier to press it in after the cookies have baked.
Refrigerate peanut butter cookie dough patties for at least an hour. Near the end of the chilling time, preheat the oven to 350°F.
Bake in the center rack for 10-15 minutes until golden brown on the sides, but still soft on the inside. Do not overbake.
Allow cookies to cool for 5 minutes, and then carefully press a Hershey's kiss in the centre of each cookie. Allow cookies to fully cool before storing.