Chopped, toasted hazelnuts or sprinkles, for topping (optional)
Preheat oven to 400 degrees.
In a mixing bowl whisk together flour, cocoa powder, baking powder and salt for 30 seconds, set aside.
In a separate mixing bowl, using an electric hand mixer set on medium-low speed, whip together granulated sugar, light-brown sugar, Nutella and butter until well blended, about 20 seconds. Stir in egg and vanilla.
Using a wooden spoon or rubber spatula and working in three separate batches, add 1/3 of the flour mixture alternating with half of the milk, mixing and folding just until combined after each addition and beginning and ending with flour mixture.
Divide batter among greased doughnut pans filling each about 3/4 full (I got 9).
Bake in preheated oven until toothpick inserted into doughnut comes out clean, about 7 - 9 minutes.
Remove from oven and allow to cool 5 minutes, then run a butter knife around edges to loosen and invert onto wire rack and allow to cool completely.
In a small mixing bowl, whisk together powdered sugar, 2 1/2 Tbsp Nutella, milk, and vanilla extract until smooth, then spread Nutella glaze over tops of doughnuts.
Top with sprinkles or chopped hazelnuts and allow glaze to set at room temperature.
Store in an airtight container at room temperature.