In a medium-size bowl, sift the cake flour then add the salt, baking powder, and nutmeg and mix together using a whisk and set aside.
Add the butter and sugar into a mixing bowl and beat on medium speed with a hand mixer for about two minutes.
Scrape the sides of the butter mixture then add the egg yolks and beat for one minute or until the mixture is fluffy. Scrape the sides again, then add the sour cream and beat on medium speed or until well combined.
Combine the flour mixture with the butter mixture and mix on slow speed until the cake doughnut dough comes together and is incorporated.
Scrape the dough along the sides of the bowl downwards onto itself slightly pressing it together. Cover the bowl with plastic and refrigerate for 30-45 minutes or until the dough firms up a bit.
Line a baking tray with parchment paper and spray with pan release or coat with some oil and set aside.
Fill a deep pot or frying pan with about 2 1/2-3 inches of vegetable or canola oil, and heat over medium heat. To check if the oil is hot enough, insert a thermometer suitable for frying oil (You can use a sugar thermometer), once it reaches 170°C (338°F) it’s ready.
In the meantime, while the oil is heating up, flour a working surface then transfer the chilled dough onto it and dust the top of the dough with some flour.
Using a rolling pin, roll out the donut dough to about 1/2 inch in thickness. Dip a 3-inch round cookie cutter into flour then cut out discs from the dough.
To give it its donut look, use a 1 1/4 inch round cookie cutter or large piping tip and cut out holes from the center of the donut discs.
Place the doughnuts and the cutout holes onto the lined baking pan. Score or cut using a knife along the ends (edges) of four sides of the doughnut disc.
Place 2 donuts at a time into the hot oil and fry for about 2 minutes on each side. If needed flip again one or two more times until the donuts are cooked and golden brown in color.
Using a slotted spoon, remove the donuts and allow the excess oil to drain on a plate lined with paper towels.
Repeat until all the donuts including the cut-out holes are fried. Keep an eye on the oil temperature, if it begins to get too hot, reduce the heat to medium-low then increase it if necessary.
Prepare the donut glaze: Add the icing sugar, water, vanilla extract, and salt into a bowl and whisk together until smooth.
While the doughnuts are still warm, dip the top into the glaze then place onto a wire cooling rack.
Serve and enjoy!