Preheat the oven to 350 and prepare a baking sheet with parchment paper. Set aside.
In a medium-sized bowl, combine the flour, cocoa powder, baking soda, and salt. Whisk to combine and set aside.
In a medium-sized bowl with a hand mixer or in the bowl of a stand mixer fixed with a paddle attachment, cream the butter until smooth, about 1 minute. Add in the sugar and beat until combined and it looks light and fluffy, about 3-4 minutes. Add in the egg, egg yolk, and vanilla. Mix to combine and scrape down the sides of the bowl as needed.
Slowly add the flour mixture to the butter mixture on low speed. Mix together until fully combined and scraping down the sides when needed.
Roll the cookie into small round balls, about 1 teaspoon each. Roll the balls in the powdered sugar or the granulated sugar. You want them to be fully covered in the sugar. Once covered in the sugar, bake the cookies for about 8 minutes. While the cookies are baking, unwrap the kisses.
The cookies should flatten out completely, but they might look a little underdone. That is okay. Let them cool for about 5 minutes before placing them in the Hershey Kisses. When you place the kisses in the cookies, the cookies should bubble out a little bit to form the blossom. Quickly place the cookies in the freezer for a few minutes so the kisses can settle without melting.
The cookies will last for a week in an air-tight container and in the freezer for about 3 months.