1. Sift together cake flour, baking powder, salt, nutmeg, and baking soda (if using) in a bowl.
2. In a stand mixer with paddle attachment, beat butter and sugar until combined.
3. Add egg yolks and beat until mixture lightens in color and thickens.
4. On low speed, add dry ingredients alternately with sour cream, mixing just until combined.
5. Cover dough and chill for at least 1 hour, up to 2 days.
6. Roll dough on a floured surface to 1/2 inch thickness and cut with a doughnut cutter.
7. Chill cut doughnuts for 30 minutes.
8. Heat oil to 350°F to 360°F in a deep pot.
9. Fry doughnuts in small batches, flipping once, until golden brown, 1.5 to 2 minutes per side.
10. Drain on a wire rack over paper towels.
11. Prepare glaze by mixing powdered sugar, corn syrup, salt, vanilla, and hot water/milk until smooth.
12. Dip warm doughnuts in glaze, allowing excess to drip before serving.