In a bowl, sift together the cake flour, baking powder, salt and nutmeg.
In the bowl of a stand mixer fitted with a paddle attachment, beat the butter and sugar together until sandy. Add the egg yolks and mix until thick and pale yellow in color, about 3-4 minutes.
Add the dry ingredients to the mixing bowl in 3 additions, alternating with the buttermilk, starting and ending with the dry ingredients, scraping the sides of the bowl down as necessary. The dough will be sticky.
Cover with plastic wrap and chill for 1 hour.
On a floured surface, roll out the chilled dough to about ½ inch thick. Use a donut cutter or two differently sized round cutters to cut out 3-4 inch doughnuts, dipping the cutters into flour as necessary to prevent sticking.
Pour the canola oil into a heavy bottomed pot to at least 2 inches deep. Insert a thermometer and heat to 375°F over medium high heat.
Add the donuts to the heated oil a few at a time, careful not to overcrowd the pot. Fry on each side about 2 minutes, but watch to make sure they don’t burn. Doughnut holes will fry about 1 minute on each side.
Remove with a slotted spoon and set on paper towels or a paper bag to soak up excess grease.
Fry remaining doughnuts, adjusting heat as needed to maintain 375°F.
To make the glaze:
Mix all ingredients in a bowl with a whisk until smooth. Dip each donut into the glaze, making sure they are covered completely. Place on a wire rack above a sheet pan to catch any excess glaze. Let sit for 20 minutes until glaze is set.
Store in an air tight container at room temperature for a few days.