Preheat oven to 350°F.  Spray a donut pan well with non-stick cooking spray and set aside.

In a large mixing bowl, mix together the flour, baking soda, and salt.  Set aside.  In a separate mixing bowl, mix together the buttermilk, Nutella, brown sugar, butter, egg, and vanilla extract until fully combined.  Add the dry ingredients to the wet ingredients and mix until just combined, making sure not to over mix the batter.  Spoon the batter into the donut cavities, make sure to only fill them about 2/3 – 3/4 of the way full.  You should end up with about 9-10 donuts, if you have a 6-count donut pan you will need to bake in two separate batches.

Bake at 350°F for 9-10 minutes or until a wooden toothpick inserted into the donut comes out clean.  Remove from the oven and cool in the pan for about 5 minutes, then remove the donuts and transfer them to a wire rack to cool.

Add the powdered sugar, Nutella, and 2 tablespoons of milk to a mixing bowl and mix until well combined.  If the glaze is too thick, slowly add one more tablespoon of milk as needed to thin out the glaze and mix until everything is well combined.  If the glaze is too thin, add in a bit more powdered sugar to thicken it up.  If the glaze is too thick, add in a bit more milk to thin it out a little.

Place a piece of foil under the wire rack (to catch any glaze that falls off the donuts).  Take each donut and dip the tops of each one into the prepared glaze, then transfer back to the wire rack for 10-15 minutes to let the glaze harden.