• Sift the flour, cornstarch, baking powder, salt and nutmeg together, 2 times. Since we're creating DIY cake flour, sifting is essential. Set aside.
  2 cups + 3 tablespoons all-purpose flour (total 330g)  , ¼ cup + 1 tablespoon cornstarch (total 35g), 2 tsp baking powder, 1 tsp salt, ¼ tsp ground nutmeg
• In the bowl of a stand mixer or using a hand mixer, beat the butter and sugar together on medium speed for 1 minute.
  ½ cup (99g) granulated sugar, 2 Tbsp (28g) unsalted butter
• Beat in the egg yolks on medium speed for 2 minutes.
  2 large egg yolks
• Beat in the sour cream and vanilla extract.
  2/3 cup (160g) sour cream, 1 tsp vanilla extract
• Gradually add the dry ingredients, mixing on low speed only until just combined. Stop as soon as you no longer see streaks of flour, overmixing at this stage leads to tough, dense donuts instead of a tender crumb.
• Cover the bowl with plastic wrap and refrigerate for at least 1 hour, until firm and not tacky. Properly chilled dough should feel stiff but not dry, similar to a soft cookie dough, and should hold its shape without sticking to your hands. The dough can be stored like this for up to 24 hours.
• Lightly flour the dough and your work surface, then roll out the dough to ½-inch thickness.
• Dip a 3-inch round cookie or biscuit cutter in flour and use it to cut circles from the dough. Then cut a hole in the center of each circle with a 1 ¼-inch round cutter (you can also use the large end of a piping tip). Arrange the donuts and donut holes on a tray lined with parchment paper or a silicone mat. Place the donuts in the refrigerator while you heat the oil.
• Pour oil into a large, heavy-bottomed pot to a depth of 2 inches, measuring after the oil is poured in to confirm the depth. The donuts should float freely once they hit the oil, not sit on the bottom or barely submerge. Heat over medium heat until a deep-fry or candy thermometer reads 325°F. Line a large baking sheet with paper towels.
  Canola oil
• Gently add 2 to 3 donuts to the hot oil at a time. Fry for 1 ½ -2 minutes, allowing the donuts to sink initially before they float to the surface (this should happen within 10 to 20 seconds). Use a slotted spoon or chopstick to flip the donuts and fry for an additional 1 ½-2 minutes. Use a slotted spoon or spider strainer to remove the donuts and place them on the prepared baking sheet to drain. Adjust the heat as needed to maintain the oil temperature at 325°F. Fry donut holes for about 30 seconds per side.

Glaze

• In a small bowl, combine the powdered sugar, milk, and vanilla extract until pourable. Add more powdered sugar if it’s too runny or more milk if it’s too thick.
  1 ½ cups (169g) powdered sugar, 1-2 Tbsp milk, 1 Tbsp vanilla extract
• Transfer the glaze to a shallow dish. Donuts are fragile straight out of the fryer, so handle them gently, a wide spatula or fork works better than your fingers for lowering them into the glaze. Dip one donut at a time, allowing the excess to fall off into the bowl. I allowed my donuts to cool for 30 minutes after frying and before glazing.