Make the doughnuts:
Preheat the oven to 425°F and spray the mini-doughnut pan with cooking spray.
In a large bowl, sift together the flour, sugar, baking powder and salt. Whisk in the buttermilk, vanilla, eggs and melted butter just until combined.
Fill each pan cavity approximately 1/3 full (about 1/2 Tablespoon per cavity). Tap the pan on the counter to release any air bubbles and level out the batter.
Bake the doughnuts for 4 to 6 minutes, or until the tops of the doughnuts spring back when touched.
Let the doughnuts cool in the pan for 5 minutes then transfer them to a cooling wrack to cool completely while you make the Nutella glaze.
Make the Nutella glaze:
In a medium bowl, whisk together the Nutella and heavy cream until combined. If the glaze is too thick, whisk in an additional 1 to 2 tablespoons heavy cream.
Dip the tops of the doughnuts into the glaze, shake off any excess (or use a spoon to smooth on the glaze) and immediately sprinkle them with chopped nuts, sprinkles or other garnishes.