Instructions

1. Combine the dry ingredients. Sift the cake flour and baking powder into a large bowl. Whisk in the salt then set aside.
2. In the bowl of a stand mixer fitted with the paddle attachment, beat the butter and the sugar on medium-low speed until smooth and creamy– about 1 minute
3. Add the egg yolks and increase the speed to medium. Continue to beat until the ingredients are well combined, about 1 minute, scraping the sides and bottom of the bowl as needed.
4. Reduce the mixer speed to low. Add 1/3 of the flour mixture. Mix on low speed until just combined, them, add half the sour cream and mix on slow speed until just combined. Repeat, adding half of the remaining flour mixture, followed by the remaining sour cream, and finally the remaining flour mixture, combining until no streaks of flour remain. Stop the mixer to scrape the sides and bottom of the bowl as needed.
5. Lightly oil a large bowl and transfer the dough to the bowl. cover the bowl with plastic wrap and refrigerate for 1 hour so the butter can firm up.
6. Generously flour a baking sheet. Flour a large piece of wax paper and tip out the dough onto the wax paper (it will be sticky!). Generously flour the top of the dough and place a second piece of wax paper on top. Roll out the dough until it is 1/2 to 3/4 inch thick. Carefully remove the top piece of wax paper.
7. Generously coat a 2 1/2 inch doughnut cutter with flour. Cut out the doughnuts coating the cuter with flour between each cut. Use a spatula to carefully move the doughnuts to the floured baking sheet. Gently brush off any excess flour on each doughnut. Gather the scraps, gently knead and repeat.
8. Line a plate with paper towels. Place a cooling rack on a baking sheet lined with parchment paper.
9. Heat 2 inches of oil in a Dutch oven or large pot for deep-frying. When the oil reaches 325°F, gently place 3 doughnuts into the oil (the doughnuts need plenty of  room to bob around). They will immediately sink and then should rise to the surface. When they rise, gently flip them over using a skewer, chopstick, or butter knife. Fry until they are golden, 2 to 3 minutes, then flip and fry on the other side until golden, another 2 to 3 minutes. Transfer the doughnuts to the rack set over the baking sheet and drain for about 1 minute.
10. Make the glaze. Sift the confectioners sugar into a large bowl. Whisk int he butter vanilla and milk until smooth.
11. While they’re still hot submerge the ridged top-side of each doughnut into the glaze and place on the cooling rack, glazed side up. Repeat until all the doughnuts have been fried and glazed. Serve immediately.