In a large bowl add 3 tablespoons of unsalted softened butter along with 1/4 cup granulated sugar, and 1/4 golden brown sugar. Mix it really well using an electric hand mixer.
Then add 2 large egg yolks and 1/2 teaspoon of vanilla extract. Mix it again once more.
Now, in a separate bowl sift together these dry ingredients: 2 cups 1/2 cup all-purpose flour, 2 teaspoons baking powder, 1 Tablespoon cornstarch, ½ teaspoon salt, and 1/4 teaspoon ground nutmeg.
Now add 1/3 of the flour mixture to the butter and sugar mixture and begin working it in with a spatula.
Then add a third of your sour cream (so 1/2 cup at a time). Mix again.
Then add the second third of the flour mixture, mix again, then add another 1/2 cup of sour cream, mix.
Now add the last third of the flour, mix, then add the last 1/2 cup of sour cream.
Place the donut dough into a large sheet of plastic wrap and cover it tightly. Refrigerate for 1 hour or until firm.
Cut out the donuts
Now lightly dust your work surface with flour and begin rolling out the donut dough to about 1/2 inch thick. 
Now reshape the scraps, roll it out again to 1/2 inch thick, and cut as many donuts out once more. Repeat this step as needed. I got about 12 donuts.
Fry the donuts
Fill a heavy-bottomed pot with about 2 inches of corn oil and bring the oil temperature to 325 degrees Fahrenheit.
Carefully fry the donuts in batches for about 2 minutes per side. Keep a close eye on them to avoid burning them. Also, make sure to adjust the heat according to the thermometer. If it brings to climb too high, bring the heat down a notch.
Once the donuts are fried, transfer them to a plate lined with paper towels to absorb the extra oil.
Make the glaze
Now, for the glaze, mix 3 cups powdered sugar, 1/2 cup salted butter, melted, 1 teaspoon vanilla extract, and 4 Tablespoons hot water.
Dip the donuts in the glaze and let them set on a cooling rack. Then enjoy them with a cup of coffee or a glass of cold milk.
Notes
Tips for making the BEST sour cream donuts
Dust the donut or biscuit cutter with flour- This will keep the cutter from sticking to the dough.
Adding 1/3 of dry ingredients and 1/3 of sour cream at a time- This will help the ingredients incorporate fully, giving you smooth dough.
Use an instant-read thermometer- You want to monitor the oil temperature for perfect frying.
Glaze them while they're still warm- The glaze will hold better if the temperature of the donut keeps it from going solid too soon.