Preheat your oven to 350°F (175°C) and line baking sheets with parchment paper.

In a large bowl, beat the softened butter, granulated sugar, and brown sugar together until the mixture is light and creamy. Add the egg, egg yolk, and vanilla extract, and mix until fully blended.

In a separate bowl, whisk the flour, cocoa powder, baking soda, and salt until combined. Gradually add the dry ingredients to the wet ingredients, mixing until a soft dough forms.

Cover the bowl with plastic wrap and chill the dough in the refrigerator for 15-20 minutes to make it easier to work with.

Once the dough is chilled, scoop out about 1 tablespoon of dough and roll it into a ball. Repeat with the remaining dough.

Roll each dough ball in granulated sugar. If you want to use sprinkles, roll some of the dough balls in sprinkles instead of sugar.

Arrange the dough balls on the prepared baking sheets, leaving about 2 inches of space between them.

Bake for 8-10 minutes, or until the edges are firm but the centers are still soft.

Take the cookies out of the oven and immediately press a Hershey's Kiss into the center of each one. The cookies may crack slightly around the edges, which is normal.

Let the cookies cool on the baking sheet for 5 minutes, then transfer them to a wire rack to cool completely. Enjoy!