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.