To make the chocolate cookie cups:
Preheat oven to 350°F. Spray a 24-count mini muffin pan with nonstick cooking spray and set aside.
In a large mixing bowl, mix together the flour, cocoa powder, baking soda, and salt. Set aside.
In the bowl of a stand mixer fitted with the paddle attachment, or in a large mixing bowl using a hand-held mixer, beat the butter, brown sugar, and granulated sugar until light and fluffy. Mix in the egg and vanilla extract, making sure to mix well after each ingredient. Slowly mix in the dry ingredients and mix until just combined, scraping down the sides of the bowl as needed.
Evenly distribute the cookie dough between all 24 mini muffin cavities. Bake at 350°F for 11-13 minutes or until done in the center. Remove from the oven and gently press down the center of each cookie cup using the bottom of a measuring teaspoon. Allow the cookie cups to cool in the pan for 5-10 minutes, or until they’ve firmed up a little and can easily be removed from the pan. Remove from the pan to a wire rack to cool completely.
To make the Nutella frosting:
In the bowl of a stand mixer fitted with the paddle attachment, or in a large mixing bowl using a hand-held mixer, beat the butter until smooth. Add in the Nutella and mix until well combined. Next, add in the powdered sugar, heavy cream, vanilla extract, and salt and mix until fully combined.
Pipe the Nutella frosting into the cooled cookie cups as desired.