To prepare the donuts:
- Preheat the oven to 350 degrees and lightly grease a donut baking pan with cooking spray.
- In a large bowl, combine the Kerrygold unsalted 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:
- 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.