In a bowl, combine flour, cocoa and salt and set aside. In your mixer beat butter and sugar until light and fluffy – about two minutes. Add egg yolk, milk, and vanilla and mix well. Slowly add in flour mixture until combined. Cover and refrigerate dough until firm – at least one hour. Roll chilled dough into one inch balls and then into sprinkles – covering entire outside of dough. Place balls on a cookie sheet about 2 inches apart. Gently use your thumb to press down the center of the ball to make a small indentation. Place back into refrigerator for about 15 minutes. Bake at 350* until set – about 12 minutes. After removing cookies from oven, immediately place a Hershey Kiss in the center of each cookie. Remove to a cooling rack to cool. Enjoy!