In a small bowl, whisk together the flour, cornstarch, baking powder, and salt.
In a medium mixing bowl, add the butter and sugar. Use a hand held mixer and on medium speed, mix until light and creamy. Add the egg yolk and vanilla extract and continue to beat until the mix is light and creamy.
Sprinkle half of the dry ingredients over the wet ingredients and mix on low speed until just incorporated. Add the remaining dry ingredients and mix until no dry streaks remain. Note - You can also mix in the dry ingredients by hand using a sturdy wooden spoon.
Cover and refrigerate the dough for at least 30 minutes.
With 15 minutes left of chill time, preheat the oven to 350F degrees and adjust the oven rack to the middle position. Line a baking sheet with a piece of parchment paper.
Add your sprinkles to small bowls, and unwrap the Hershey Kisses.
Using a cookie scoop (I use a smaller 1.5 tablespoon #50 cookie scoop for these cookies), scoop 6 equal cookies. Roll the cookies between your hands to make dough balls. Roll the cookies through your desired sprinkles and place on the baking sheet. Note - The cookies will flatten and spread slightly. I do not recommend flattening the cookie balls because we want a puffier cookie to hold the Hershey Kiss.
Bake the cookies for 11 to 13 minutes, or until the edges are set. Once the cookies come out of the oven, immediately push a Hershey Kiss in the middle. Cool the cookies on the baking sheet set on a wire cooling rack for 5 minutes, then transfer to the cooling rack to cool completely. Enjoy!