In a medium bowl, sift together the flour, baking powder, salt, and nutmeg.

In a large mixing bowl or the bowl of a stand mixer, combine the butter and sugar. Beat together on medium speed until well combined, about 1 minute. Add the egg yolks. Beat on medium speed until light and fluffy, about 2 minutes. Scrape down the bowl. Add the sour cream and beat until well combined.

Add the flour mixture. Beat on low speed until well combined, scraping down the bowl once to make sure everything is incorporated. Scrape the dough down into the bowl, cover, and refrigerate for at least 1 hour or until firm, and up to 1 day.

Fill a large heavy-bottomed pot with oil 2 inches deep. Place over medium heat until a deep-fry thermometer registers 335F. Line a large baking sheet with a paper bag or paper towels.

While the oil is heating, invert the chilled dough onto a well-floured surface. Dust the dough well with flour. Roll to ½-inch thickness. Using a 3-inch round cookie or biscuit cutter dipped in flour, cut circles out of the dough. Cut holes in the center of each circle using a 1 ¼-inch round cutter. (The large end of a piping tip works great, too!) Place the donuts and holes on the parchment-lined tray.

Carefully place 2 to 3 donuts in the hot oil at a time. Fry for 2 minutes. (They will sink to the bottom at first but should float in about 5 to 10 seconds.) Flip and fry for another 2 minutes. Flip one more time and fry for 30 to 60 seconds or until golden brown. Remove the donuts with a slotted spoon or strainer and let drain on the paper. Adjust the heat as necessary to keep the oil between 330F and 340F.

While the donuts are warm, sift the confectioners’ sugar into a medium bowl. Whisk in the water, the vanilla, and salt until smooth.

Dip the craggy top of each donut in the glaze. Place on a wire cooling rack, glaze side up, and let cool for a few minutes. Serve warm or let cool completely. Leftover donuts can be stored, covered, at room temperature for up to 3 days.