For the Donuts:

1. In a medium sized mixing bowl combine cake flour, baking powder, salt and baking soda. Set aside.
2. In the bowl of a stand mixer fitted with a paddle attachment combine sugar, butter and shortening. Beat on a medium-high speed until smooth. Add in egg yolks and mix until just incorporated.
3. Add sour cream and vinegar and mix to combine. Add in the flour mixture and mix until just until combined. Transfer the dough into a greased bowl and cover with plastic wrap. Chill for 30 minutes in the refrigerator.
4. In the meantime, in a large pot, heat the oil over medium heat until it reaches 375°F.
5. Turn the chilled dough out onto a clean, well-floured surface. Press the dough out into a ¾” thick. Use a donut cutter to cut out as many donuts as you can. Form the remaining dough back together and repeat the process until all the dough has been used.
6. To fry the donuts, carefully place 2-3 donuts at a time into the preheated oil and fry for 2 minutes per side. Flip and fry the other side for 2 minutes. Once fried, carefully remove the donut from the pot and place into a cooling rack.

For the Glaze

1. In a large mixing bowl combine powdered sugar, milk, corn syrup, vanilla and salt. Whisk
   together until smooth.
2. Dip the warm donuts into the glaze, then place into a wire cooling rack to let the excess drip off.