Step 1: Preheat oven to 350°F (175°C). Line baking sheets with parchment paper.
Step 2: In a medium bowl, whisk together flour, cocoa powder, baking soda, and salt. Set aside.
Step 3: In a large bowl, cream together the softened butter, granulated sugar, and brown sugar with an electric mixer until light and fluffy.
Step 4: Beat in the egg and vanilla extract until well combined.
Step 5: Gradually add the dry ingredients to the wet ingredients, mixing on low speed until just combined and a soft dough forms.
Step 6: Place powdered sugar in a shallow dish. Roll spoonfuls of dough into small, 1-inch balls. Roll each ball generously in powdered sugar to coat completely.
Step 7: Place the coated cookie dough balls about 2 inches apart on the prepared baking sheets.
Step 8: Bake for 8-10 minutes. The cookies will be slightly puffed and set at the edges but still soft in the center. Do not overbake.
Step 9: Immediately upon removing from the oven, gently press one unwrapped chocolate kiss into the center of each warm cookie. The chocolate will soften and melt slightly.
Step 10: Let the cookies cool on the baking sheet for 5-10 minutes before transferring them to a wire rack to cool completely.