Heat your oven to 350°F. Spray the bottom and sides of a 9x9-inch baking pan with cooking spray. Set aside.

In the bowl of a stand mixer with the paddle attachment or in a large bowl using an electric hand mixer, beat butter and granulated sugar on medium speed for 3 to 4 minutes or until the mixture is light and fluffy. Beat in egg and vanilla extract until just combined.

In a separate medium bowl, whisk together flour, cocoa powder, baking soda, cornstarch and salt until combined. Add to butter-sugar mixture; stir until just combined.

Using greased hands or a spatula, press dough evenly into bottom and sides of prepared baking pan (the dough will be thick). Sprinkle evenly with 1 teaspoon sparkling sugar.

Bake for 17 to 20 minutes or until edges are set and a toothpick inserted in the center of the pan comes out clean. Remove from oven; cool for 7 to 10 minutes, then gently press Hershey's Kisses into pan in 4x4 rows. Sprinkle with more sparkling sugar, if desired. Transfer pan to the fridge for 15 minutes (this important step ensures the Kisses won't melt!). Remove from fridge; finish cooling completely on a cooling rack.

Use a sharp knife to cut fully cooled bars into 4x4 rows. Store leftover bars in an airtight container at room temperature for up to 1 week.