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.