In a medium bowl, whisk together the cake flour, baking powder, salt, and nutmeg:
255 grams cake flour
1 1/2 teaspoon baking powder
1 teaspoon salt
1/2 teaspoon ground nutmeg
In the stand mixer bowl, beat the sugar and butter together until sandy. Add the egg yolks and mix until light and thick, about 3 min:
1/2 cup sugar
2 tablespoons butter (there are 8 tablespoons in a stick)
2 large egg yolks
Add the dry ingredients to the mixing bowl in 3 additions, alternating with the buttermilk, mixing until just combined on low:
1/3 cup buttermilk
Cover with plastic wrap and chill for at least 1 hour and up to 24 hours.
On a floured surface, roll out the dough to 1/2-inch thickness. Cut into doughnuts using a doughnut cutter or two different sized round cutters. (I used a cocktail shaker and jigger!)
Pour oil into large pot until its 2-4 inches. Maintain heat at 350°F. (I love using my wok for frying!)
Add the doughnuts to the oil, fry 2 min each side.
Remove with a slotted utensil, let drip, then place on paper towel to dry.