Sift dry ingredients:

In a medium bowl, combine cake flour, baking powder, nutmeg, and kosher salt by sifting them together.

Cream butter and sugar:

Using a hand mixer, beat softened butter in a large bowl for 2 minutes. Then incorporate sugar and mix for another 3 minutes. Add egg yolks and vanilla extract; blend until uniform, roughly 2 minutes.

Fold in sour cream mixture:

Fold in half the sour cream and half the flour mixture using a flat spatula until just combined. Add the remaining sour cream and flour, then mix until fully integrated. The dough should be somewhat firm.

Chill dough:

Wrap the dough in plastic and chill for at least 1 hour, or overnight, until firm.

Heat oil:

In a large pot or Dutch oven, heat vegetable oil to 350°F (175°C).

Roll and cut donuts:

On a heavily floured surface, roll out dough to a thickness of 1/2 inch. Cut donuts using a 3-inch biscuit or donut cutter, and punch a small hole in the center (a soda bottle cap works well). Gather scraps, re-roll, and repeat.

Chill cut donuts:

Arrange cut donuts on a baking sheet and refrigerate uncovered for 20 minutes.

Fry donuts:

Fry 2-3 donuts at a time in the hot oil, cooking each side for about 2-3 minutes until deep golden brown. Transfer to a wire rack or paper towels to remove excess oil.

Make glaze:

While donuts are cooling, prepare the glaze: whisk powdered sugar, corn syrup, warm milk, and vanilla extract together in a large bowl until smooth.

Glaze donuts:

After donuts have cooled, dip each side in the glaze and set on a wire rack to let the glaze harden.