- In the bowl of a stand mixer, beat together the butter and sugar until sandy with a paddle attachment. 
- Add in the egg yolks one at a time and mix until light. 
- Sift together the cake flour, baking powder, salt and nutmeg into a large bowl. 
- Add the dry ingredients into the stand mixer bowl, in 3 additions, alternating with 2 sour cream additions in the middle. (The dough will be sticky) 
- Cover the bowl with plastic wrap and place in the fridge to chill for 1 hour. 
- Sprinkle flour over a flat surface and roll out the dough to about 1/2 inch thick. 
- Use a doughnut cutter (or two different sized round cutters) to cut as many doughnuts as possible. 
- Fill a frying pan with 1 inch of oil heated to 325 degrees. 
- Shake any excess flour off the doughnuts before carefully adding them to the oil. 
- Allow the donuts to cook for about 1-2 minute then flip and fry for an additional 1 minute or so on the other side. Be careful not to let them burn. 
- Let them drain on paper towels to soak the excess grease. 
- Allow to cool for about 5 minutes before dipping in glaze. 
- Mix all glaze ingredients in a bowl, and whisk until smooth. Dip both sides of the doughnut into the glaze. 
- Place dipped donuts on a wire cooling rack over a sheet pan to catch any excess glaze. 
- Allow glaze to set for about 20 minutes and enjoy.