In a medium bowl, sift together the cake flour, baking powder, fine sea salt, and ground nutmeg. Set aside.
In a large mixing bowl fitted with a paddle attachment, beat the butter and granulated sugar on low speed until the mixture resembles damp sand.
Add the egg yolks and continue mixing until the mixture becomes lighter in color and thickens. Scrape down the sides of the bowl as needed.
Add the dry ingredients in three additions, alternating with the sour cream and beginning and ending with the flour mixture. Mix just until combined. The dough will be sticky, so avoid overmixing.
Cover the dough with plastic wrap and refrigerate for about 1 hour, or until it becomes firm enough to roll and cut easily.
Lightly flour your work surface and rolling pin. Roll the chilled dough to approximately ½ inch thick. Use a doughnut cutter or two round cutters of different sizes to cut out the doughnuts and their centers. Gather and gently reroll any scraps as needed.
Pour approximately 2 inches of canola oil into a heavy-bottomed pot fitted with a deep-fry thermometer. Heat the oil to 325°F (163°C), maintaining a steady temperature for even cooking.
Carefully fry a few doughnuts at a time, avoiding overcrowding the pot. Fry until golden brown and cracked, turning as necessary to cook them evenly. Transfer the finished doughnuts to a wire rack set over paper towels to drain.