Preheat oven to 350°Spray a muffin tin with cooking spray.
In a small bowl, whisk together flour, baking powder, sifted dutch process cocoa powder, and salt. Set aside.
Beat butter, granulated sugar, and brown sugar on medium-high speed until light and fluffy.
Add egg and vanilla extract. Beat until combined.
Add flour mixture and mix until just combined.
Divide dough with a large cookie scoop between muffin cups and press dough down to flatten.
Bake for 10-12 minutes or until mostly set, but still soft in the middle.
As soon as the cookie cups come out of the oven, use the rounded tip of a kitchen utensil or a small jar to create a well in the center of the cookie.
Allow the cookie cups to cool in the pan for 10 minutes. Loosen each slightly by twisting in the pan, then remove them from the pan and transfer to a wire rack to cool completely.
While the cookies are cooling, prepare the Nutella mousse filling. Whip heavy cream until stiff peaks form. Set aside 1/3 of the whipped heavy cream for decorating
In another bowl beat sugar and cream cheese until smooth. Add Nutella and beat until incorporated. Fold in remaining whipped cream.
Pipe Nutella mousse into each cooled cookie cup. Refrigerate until set for about 2 hours, then top with whipped cream, Nutella, and chopped hazelnuts.
Serve chilled and eat within 2-3 days.