Step 1: In a large bowl, cream together the softened unsalted butter and sugar until light and fluffy. This should take about 2-3 minutes with an electric mixer.
Step 2: Beat in the vanilla extract and the egg until well combined and smooth. Scrape down the sides of the bowl as needed.
Step 3: In a separate medium bowl, whisk together the all-purpose flour, baking soda, salt, and cornstarch.
Step 4: Gradually add the dry ingredients to the wet ingredients, mixing on low speed until just combined. Be careful not to overmix the dough.
Step 5: Cover the cookie dough with plastic wrap and refrigerate for at least 30 minutes. This chilling process helps prevent the cookies from spreading too much.
Step 6: Preheat your oven to 375°F (190°C) and line baking sheets with parchment paper or silicone baking mats. Unwrap all 24 peppermint Hershey Kisses and set them aside.
Step 7: Roll the chilled dough into 1-inch balls. Place them about 2 inches apart on the prepared baking sheets.
Step 8: Bake for 10-12 minutes, or until the edges are lightly golden and the centers are still soft. Do not overbake.
Step 9: Immediately upon removing the cookies from the oven, gently press one unwrapped peppermint Hershey Kiss into the center of each warm cookie.
Step 10: Let the cookies cool on the baking sheets for 5 minutes before transferring them to a wire rack to cool completely. Enjoy your festive Peppermint Hershey Kiss Sugar Cookies!