Preheat the oven to 350F. Spray a donut pan with non-stick cooking spray and set aside. In a large bowl, whisk together the flour, baking powder, baking soda, nutmeg and salt until combined, then set that aside.
In a separate bowl, whisk together the egg, brown sugar, milk and yogurt (or sour cream) until smooth. Whisk in the melted butter and vanilla until fully incorporated. Pour the wet ingredients into the dry ingredients and stir until just combined. Do not overmix.
Now, spoon the batter into the donut pans cavities. Per the recipes recommendation, I placed the batter into a large Ziploc bag, cut a hole at one corner and piped the batter into the donut cavities. Fill no more than 3/4 of the way full with batter.
Bake for 8-10 minutes, or until the edges are lightly browned. After about 2 minutes out of the oven, transfer the donuts to a wire rack and bake the remaining batter. Allow the donuts to cool enough to handle before glazing.
To make the nutella glaze, place the glaze ingredients in a medium bowl and whisk them together. Start with just 4 tsp. of milk and add more if you'd like the consistency to be thinner.
Dip the tops of the donuts into the nutella glaze. If using sprinkles as well, then immediately sprinkle them over the freshly glazed donuts. Let the donuts sit out on a wire rack until the glaze sets, about 15-20 minutes. Any leftover donuts will be good stored in an airtight container for another day or two.