In a medium bowl, whisk together cake flour, baking powder, salt, and nutmeg.
In a large mixing bowl or bowl of a stand mixer, cream together the butter and sugar until sand-like consistency, about 1 minute.
Add egg yolks and beat until well combined. Scrape down the sides of the bowl.
Add dry ingredients in 3 additions, mixing on low, just until combined. Alternate with sour cream and scraping down the sides of the bowl in between additions.
The dough should be smooth and slightly sticky. If it’s too sticky, add 1 tablespoon of cake flour.
Cover bowl with plastic wrap and chill in the refrigerator for at least 1 hour or until firm.
ROLL OUT AND CUT DOUGH:
Roll the dough out to ½ inch thick on a floured surface.
Use a donut cutter with a hole in the center, dip in flour to prevent sticking, and cut out donuts.
Place the cutout dough on a parchment paper-lined baking sheet and place it in the refrigerator while the oil is heating. The dough should be slightly cold before frying.
FRY DONUTS:
To fry the donuts, add 2-3 inches of oil into a heavy-bottomed pan with a thermometer attached and heat to 325 degrees F.
Fry donuts a few at a time for 2-3 minutes on each side, depending on the size of your pan.
Be sure to monitor the thermometer and adjust the temperature if needed.
Remove donuts from the pan when golden brown and place them on a wire rack to drain.
MAKE GLAZE:
Whisk together all ingredients until smooth in a medium bowl.
GLAZE DONUTS:
While the donuts are still warm, submerge the top side of the donuts into the glaze and place them on a cooling rack, glaze side up.
Notes
The donuts are best served the same day they are made.