In a bowl, sift together the cake flour, baking powder, salt, pumpkin spice, and nutmeg powder.

In the bowl of a stand mixer that's been fitted with the paddle attachment, beat the butter and the sugar together for a couple of minutes until it becomes sandy. Add the yolks one at a time while the mixer is on the stir setting. Add the pumpkin puree and mix until combined. Add the dry ingredients to the mixing bowl in 3 additions, alternating with the sour cream (in 2 additions), starting and ending with the flour. The dough with be super sticky. Cover the bowl with plastic wrap and allow it to chill in the refrigerator for 1 hour.

On a well floured surface, dump out the dough and roll it into a ½ inch thickness. Using a doughnut cutter, cut out the donuts, dipping the cutter in flour as necessary to prevent dough from sticking. Depending on the size of the donut cutter you use, you will end up with 18-24 doughnuts/holes.

Heat 2 inches of canola oil in a 12 inch cast iron skillet or a dutch oven. Attach the thermometer or monitor the temperature (by increasing/decreasing the flame) so it stays at 325ºF throughout the frying process. Fry the doughnuts a few at a time, be careful not to overcrowd the skillet as this will cause temperatures to drop quickly. Fry on each side for about 1 ½ minutes, a larger cutter may require you to fry the donuts slightly longer. Do not allow the donuts to burn. Drain on a paper towel lined plate.

Mix the ingredients for the glaze in a medium bowl with a form or whisk. Dunk the doughnuts one at a time into the glaze. Place on a wire rack that's been set above a baking sheet to catch dripping glaze. Let glaze set for 20 minutes. Doughnuts will taste best when freshly made, leftovers can be store in an air tight container that's kept at room temperature for 2-3 days.