Remove Hershey's kisses from wrappers. Do this a few day ahead and those them at room temperature or unwrap them weeks ahead and freeze them for up to 6 months. (This is a great job for kids).
Heat oven to 350° F
Spray a sheet pan with nonstick cooking spray (or use parchment paper or a Silpat Baking Mat).
In a medium mixing bowl, whisk together flour, cocoa powder, baking soda, and salt. Set aside.
Using a standing mixer or hand mixer, beat butter until smooth. Add both sugars and beat on high until light and fluffy.
Add vanilla extract, eggs and milk and mix on high until smooth.
Add dry ingredients and mix on low until combined.
Roll dough into 1 inch balls (be sure to keep these small so that you have a good ratio of cookie to Hershey's Kiss in he finished cookie).
Place cookies on prepared sheet pan, leaving a couple inches between each.
Bake until cookies are just barely set (they will still look very soft), 9-10 minutes.
As soon as cookies come out of the oven, gently press a Hershey's Kiss into the top of each cookie.
Let cool on sheet pan for a couple minutes before transferring them to a sire rack to cool completely.