Step 1

Sift the flour, baking powder, salt, and nutmeg together into a mixing bowl, and set aside.In a stand mixer fitted with the paddle attachment, mix the sugar and shortening for 1 minute on low speed, until sandy. Add the egg yolks, then mix 1 more minute on medium speed, scraping the sides of the bowl with a rubber spatula if necessary, until the mixture is light colored and thick.Add the dry ingredients to the wet ingredients in three separate additions, alternating with the sour cream, mixing until just combined on low speed and scraping the sides of the bowl each time. The dough will be sticky, like cookie dough.

Step 2

Transfer the dough to a clean bowl and refrigerate, covered with plastic wrap, for 45 minutes (or up to 24 hours).Using a candy thermometer to measure the temperature, heat oil (at least 2 inch deep) in a deep fryer, large pot, or high-sided frying pan to 325°F. Roll chilled dough out on a generously floured counter or cutting board to 1/2 inch thick, or about 8 inches in diameter, flouring the top of the dough and the rolling pin as necessary to prevent sticking. Cut into as many doughnuts and holes as possible, dipping the cutter into flour before each cut. Fold and gently reroll the dough (working with floured hands makes the dough less sticky), and cut again.Shake any excess flour off the doughnuts before carefully adding them to the hot oil a few at a time, taking care not to crowd them. Once the doughnuts float, fry for 15 seconds, then gently flip them. Fry 75 to 90 seconds, until golden brown and cracked, then flip and fry the first side again for 60 to 75 seconds, until golden. Drain a rack set over paper towels/absorbent paper.

Step 3

Glaze:Place the ingredients in a large mixing bowl or in the work bowl of a stand mixer fitted with the paddle attachment. Using a whisk, or with the machine on low speed, blend until the mixture is smooth and all the sugar has been incorporated, scraping the sides of the bowl with a rubber spatula, if necessary. If the glaze seems to thick, add more hot water, a teaspoon at a time.To glaze, dip one side of each hot doughnut into the warm glaze, and let dry 10 to 15 minutes before serving.