Next, make the cookie dough. Preheat the oven to 350 degrees Fahrenheit (180 degrees Celsius) and line a baking sheet with parchment paper.
In a medium-sized bowl, whisk together the flour, dark cocoa powder, cornstarch, baking powder and salt. Set aside.
In the bowl of a stand mixer fitted with the paddle attachment, or using a handheld mixer, beat the butter and cream cheese together on medium-high speed until smooth and no lumps remain. Add the granulated sugar and beat on medium-high speed until combined, about 2 minutes.
Add the egg, egg yolk, and vanilla extract, and mix on low-medium speed until combined, scraping down the bottom and sides of the bowl as necessary. Add the dry ingredients and mix on low speed until a dough forms. It will be thick and sticky.
Fill a small bowl with granulated sugar and another small bowl with powdered sugar.
Using a small cookie scoop (mine was a ¾ oz cookie scoop), approximately 25 grams of dough per cookie, scoop the cookie dough and immediately place it in the granulated sugar. Toss to coat - immediately placing the scooped cookie dough into the granulated sugar will prevent the dough from sticking to your hands.
Roll the dough into a ball and toss in the powdered sugar - coat it generously. Place on the lined baking sheet, spacing the cookies about 2-3 inches apart. The cookies will spread a little in the oven.
Bake the cookies for 11-12 minutes until the cookies are set. Remove the cookies from the oven and immediately, using a rounded teaspoon, press the bottom of the teaspoon into each cookie until it forms a rounded indent.
Let the cookies cool on the baking sheet for 5 minutes, then transfer to a cooling rack to cool completely. Once cool, fill the indent in the cookies with the caramel. If the caramel has cooled a lot and has become thick, microwave it for 7-10 seconds to loosen it up. This will make it easier to spoon the caramel into the cookies. Dust with additional powdered sugar if desired. Enjoy!
Store in an airtight container in the fridge.