In a large mixing bowl, use an electric mixer to beat the softened butter, granulated sugar, and brown sugar until smooth and creamy.
Add the egg yolks and beat until well combined.
Stir in the vanilla extract and sour cream, making sure to scrape down the sides of the bowl.
Combine Dry Ingredients
In a separate bowl, whisk together the flour, baking powder, cornstarch, salt, and nutmeg (if using).
Gradually add the dry ingredients to the wet mixture, stirring gently by hand with a spatula until just combined. Avoid overmixing to keep the donuts tender.
Chill the Dough
Cover the dough with plastic wrap and refrigerate for at least 1 hour.
Shape the Donuts
Lightly flour a clean surface and roll out the chilled dough to about ½-inch thickness.
Use a donut cutter (or a large round cutter with a small round cutter for the center) to cut out the donuts and holes.
Gather the dough scraps, re-roll, and continue cutting until all dough is used.
Place the cut donuts on a wax paper-lined baking sheet and refrigerate for 15 minutes.
Heat the Oil
In a deep, heavy-bottomed pot or Dutch oven, heat at least 2-3 inches of vegetable oil to 350-360°F (175-185°C). Use a candy thermometer to monitor the temperature.
Fry the Donuts
Once the oil reaches the correct temperature, carefully add 3-4 donuts at a time using tongs.
Fry for 1 to 1 ½ minutes per side, flipping once, until golden brown.
Remove and place on a paper towel-lined plate to drain excess oil.
Prepare the Glaze
In a small bowl, whisk together the powdered sugar, melted butter, vanilla, and hot water until smooth.
Glaze the Donuts
Dip each warm donut into the glaze, then transfer to a wire rack to set.
Enjoy warm!
Notes
Cooking Time: 3 minutes per batch
Avoid overmixing the dough to keep the donuts soft.
Chilling the dough helps prevent excess oil absorption when frying.
If the oil is too cold, donuts will absorb too much oil. If too hot, they will brown too quickly before cooking through.