In a small bowl sift together the cake flour, baking powder, salt and nutmeg.
In a large mixing bowl beat the butter and sugar together until well combined (the texture will be sandy). Add the egg yolks and mix well. Add the dry ingredients in three batches, alternating with the sour cream, and ending with flour. Wrap the dough in plastic wrap and chill for 1 hour.
When the dough is almost finished chilling, make the glaze by whisking together all ingredients in a large shallow bowl.
Remove the dough from the fridge and roll it out on a floured surface to about 1/2 inch thickness. Use a donut cutter (or two round biscuit cutters) to cut as many donuts out as possible. You should get around 12.
Pour about 2 inches of canola oil into a heavy bottomed pot. Heat the oil to 325° F. Fry the donuts 2-3 at a time, being careful not to overcrowd the pot. Fry for about 2 minutes on each side, being careful not to let them burn. Remove them to a plate lined with paper towels to soak up the grease.
Place a wire cooling rack above a sheet pan. Dip donuts in the glaze, covering on each side, and place on the wire rack. Let sit for 15-20 minutes until glaze is set.
Donuts are best served immediately, but you may store them in an air tight container at room temperature for a few days.
Use different sized cutters for larger/smaller donuts