To prepare the donuts:

1. Preheat the oven to 350 degrees and lightly grease a donut baking pan with cooking spray.
2. In a large bowl, combine the butter, eggs, sour cream, and vanilla extract. In a separate bowl, combine the flour, cocoa powder, brown sugar, espresso powder baking powder, baking soda, and salt. Pour the wet ingredients into the dry ingredients and stir to combine. The mixture is thick, so be sure to incorporate the ingredients well. Pour the batter into a piping bag or a quart-sized plastic bag with the end snipped off. Pipe the mixture into the wells of the donut pan, filling each about ¾ of the way full. Bake in the preheated oven for about 10 minutes or until a toothpick inserted comes out clean. Repeat the process with the remaining batter until all the donuts are cooked. Set aside to cool.

To prepare the glaze:

1. Whisk all of the ingredients together. Add additional milk to thin out the mixture if desired. Dip the cooled donuts in the glaze and allow to set. Feel free to sprinkle with chopped toasted hazelnuts prior to glaze setting if desired.