In a medium bowl, whisk together flour, cocoa powder, salt and baking soda. In a large bowl using a mixer, beat softened butter with sugar until fluffy.
Add in egg and vanilla extract, beating until smooth. Add the flour mixture in three separate additions, beating after each until all of the dry ingredients are fully incorporated. Cover and refrigerate the dough for at least one hour.
Preheat oven to 350 F. Roll cookie dough into balls and place 2-inches apart onto ungreased baking sheets. Bake for 10-12 minutes. Remove from oven and place one chocolate kiss on each cookie.
