Preheat the oven to 350°F and line a cupcake pan with paper liners. Set aside.
Whisk together the 1½ cups all-purpose flour, 1 cup granulated sugar ½ cup Hershey's Special Dark cocoa powder, 1 teaspoon baking powder, ½ teaspoon baking soda, and ¼ teaspoon salt in a large mixing bowl.
Add ½ cup unsalted butter (make sure it's very soft and you can easily press into it with a finger) to the dry ingredients and reverse cream until the mixture looks sandy.
Beat in the 2 large eggs, ½ cup whole milk, and 1 teaspoon vanilla extract into the butter mixture and mix until combined.
Fill the muffin cups about ¾ of the way full with the batter.
Bake for 18 to 20 minutes or until a toothpick inserted in the center comes clean.
Allow the cupcakes to cool for a few minutes in the pan, then transfer them to a wire rack to cool for about 30 minutes.
Once the cupcakes are cooled, use a cupcake corer, a frosting piping tip, or a knife to cut a small hole in the top center of each cupcake. Be careful not to cut all the way through the bottom.
Fill each hole with some of the 1 cup Nutella using a small spoon or a piping bag. Give each cupcake a small tap on the counter to make sure the Nutella settles into the hole.
To make the frosting, cream together 1 cup unsalted butter in a large mixing bowl until creamy.
Add the 4 cups powdered sugar, 1 cup at a time, mixing between each addition until mostly incorporated.
Beat in 1 cup Hershey's Special Dark cocoa powder¼ cup heavy cream and mix until smooth.
Frost the cupcakes with the chocolate frosting, and sprinkle finely chopped hazelnuts on top.