Preheat oven to 325 degrees F.
Combine flour, cocoa, baking powder and salt (if using) in a bowl and set aside.
In a mixing bowl, using a hand-held electric mixer, beat butter, Nutella and both sugars until smooth. Beat in the egg and vanilla. By hand, stir in the flour mixture. When flour is incorporated, stir in the chips and nuts (if using). Using an even, packed 1/2 teaspoon, shape the dough into small balls and arrange about 20 to a sheet on an ungreased baking sheet. Sprinkle tops with Halloween sprinkles if using.
Bake for 12 minutes, but keep an eye on them. My cookies always take 12 minutes, but ovens vary. Plus it's hard to tell when these guys are done because they're so dark.