Preheat the oven to 350ºF. Grease a donut pan and set it aside.
Mix the dry ingredients and set aside.
In a separate bowl, beat together the wet ingredients. Add to the wet ingredients into the dry ingredients and stir it together until combined.
Scoop the batter into a plastic bag or pastry bag. If using a plastic bag, snip the corner off to create a makeshift pastry bag. Pipe the batter into the donut pan and then spread it out evenly with a spatula or your fingers. Bake on the center rack for 18 minutes.
While the donuts are baking, melt together all the icing ingredients until smooth.
Allow the donuts to cool for a least 10 minutes in the pan, then turn out onto a cooling rack and cool completely.
Drizzle with icing and sprinkle with chopped hazelnuts if desired.