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 scrape 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.

On a floured surface, roll the dough out to ½ inch thick.

Use a donut cutter with a hole in the center, dip in flour to prevent sticking, and cut out donuts. 

Place on a parchment paper-lined baking sheet and place in the refrigerator while the oil is heating. The dough should be slightly cold before frying. 

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 keep an eye on the thermometer and adjust the stove temperature if needed.

Lay a paper bag onto a flat surface.

Remove donuts from the pan when they are golden brown and place them on a wire rack to drain.

Glaze

Whisk together all ingredients until smooth.

While donuts are still warm, submerge the top side of the donuts into glaze and place them on a cooling rack, glaze side up. 

Donuts are best served on the same day.