Preheat oven to 350°F and line a baking sheet with parchment paper. In a medium bowl, whisk together flour, cocoa powder, baking soda, and salt. In a large bowl, cream softened butter, granulated sugar, and brown sugar until light and fluffy. Add egg and vanilla extract and mix until combined. Stir in milk. Gradually add dry ingredients to wet mixture and mix until dough forms. Cover and chill dough for at least 30 minutes. Roll dough into 1-inch balls and roll each in granulated sugar. Place on prepared baking sheet. Bake for 8–10 minutes until edges are set and tops crack slightly. Remove from oven and immediately press a chocolate kiss into the center of each cookie. Cool on baking sheet for 5 minutes before transferring to a wire rack.