Several hours before, to days before, freeze your Hershey's kisses.
In a medium bowl or the bowl of your stand mixer, add butter and sugar and cream together until light and fluffy (about 3 minutes).
Add in egg yolk and mix until homogenous.
Add milk and vanilla and combine into the wet ingredients.
Add in your cocoa powder, mixing until cookie dough is dark and richly colored.
Add in flour and salt and stir in until just combined.
Using a cookie scoop (I used a 2½ tbsp scoop), scoop all of the dough onto a lined cookie sheet. Place the cookie sheet in the refrigerator for at least one hour.
Once cookie dough is sufficiently chilled, measure out 2-4 tbsp of granulated sugar into a small bowl.
Preheat the oven to 350°F
Take one scoop of dough and roll into a ball between your palms. Once it is round, place into the sugar bowl and cover the exterior in sugar. Place the dough ball back onto a lined cookie sheet.
Repeat with the rest of the cookie dough until all are covered in sugar. Make sure the dough balls are spaced with at least 2-3" of space between them.
Bake the cookies for 10-12 minutes. The cookies will have spread a small amount but still be somewhat domed.
While the cookies are baking, remove your kisses from the freezer and unwrap them.
Immediately after taking cookies out of the oven, press a Hershey's kiss into the center of each cookie. You want to press the kiss down slightly which will cause the cookie to spread a little, but you do not need to press it all the way through the cookie to the pan.
Allow the cookies to set on the baking sheet for about 10 minutes before moving them to a wire rack to cool.
While the cookies cool, the Hershey's kisses will soften completely so before storing make sure the cookies have cooled completely and the chocolates have re-solidified. Cookies will keep in an airtight container for several days.
