In a large bowl, whisk together cake flour, baking powder, salt, and nutmeg. Set aside.
2 ½ cups (320g) cake flour, 1 ½ teaspoons baking powder, 1 teaspoon salt, ½ teaspoon ground nutmeg
Beat sugar and butter together in a stand mixer until sandy.
½ cup granulated sugar, 2 Tablespoons salted butter
Add egg yolks, one at a time, mixing until light. Add vanilla.
2 egg yolks, 1 teaspoon pure vanilla extract
Add half of the flour mixture to sugar mixture, mixing just until combined. Add ½ of sour cream, mixing just until combined again. Repeat with remaining flour and sour cream to create a sticky dough. Cover with plastic wrap and chill in the fridge for 1 hour.
½ cup sour cream
Sprinkle a clean surface with flour, then roll out chilled dough to ½-inch thick.
Cut out doughnuts using a 3-inch circle cutter and 1 1/4-inch round cutter or large piping tip for the centers. If your dough gets sticky, dip your cutters into flour or powdered sugar before pushing into the dough.
Fill a large skillet or dutch oven with 1 to 2 inches of oil. Heat over medium-high heat until oil reaches between 325°F and 350°F when tested with a thermometer.
Canola oil
Working in batches so as not to crowd the pan, shake off excess flour from each doughnut, then fry for 1-2 minutes on one side. Flip and fry for 1 minute on the other side until pale golden brown and cooked through.
Carefully remove doughnuts with tongs or a slotted spoon and drain on paper towels to absorb excess oil. Cool 5 minutes before dipping in glaze.
Whisk glaze ingredients together in a medium bowl. Dip one or both sides of each doughnut in glaze, then set on a wire rack. Let glaze set for 20 minutes before serving.
3 cups (360g) powdered sugar, 1 teaspoon pure vanilla extract, ⅓ cup very hot water, Pinch of salt