01 -
In a medium bowl, whisk together cake flour, baking powder, salt, and nutmeg until evenly combined.
02 -
Beat sugar and softened butter until sandy. Add egg yolks one at a time, then mix in vanilla. Add the dry ingredients in two additions, alternating with the sour cream, and mix just until a sticky dough forms.
03 -
Pat the dough into a thick disk, wrap tightly, and refrigerate for at least 1 hour or up to overnight.
04 -
On a lightly floured surface, roll the chilled dough to 1/2 inch thick. Cut into rounds with a 3-inch doughnut cutter, then cut out the centers. Gather scraps gently and cut more doughnuts or holes.
05 -
Pour oil into a heavy pot to a depth of about 2 inches. Heat to 325-340 F and set a wire rack over a baking sheet for draining.
06 -
Fry 2-3 doughnuts at a time for 1-2 minutes on the first side and about 1 minute on the second side, until cracked and pale golden. Transfer to the rack and cool for 5 minutes.
07 -
Whisk powdered sugar, hot water, vanilla, corn syrup if using, and salt until smooth. Dip warm doughnuts into the glaze, return to the rack, and let set for 15-20 minutes.