In a medium-size mixing bowl, whisk cake flour, cornstarch, baking powder and salt together; set aside.
In a stand mixer, cream the sugar and butter on medium speed for about a minute. Add in the egg yolks one at a time, letting them fully incorporate between the additions; beat until light and fluffy, about two minutes. Turn the speed to low, add the sour cream and slowly stream in the milk. Working 1/2 cup at a time, add in the dry ingredients until fully incorporated.
While the donuts are cooling, make the glaze: Sift the powdered sugar into a medium bowl. Add the milk and salt, and whisk until smooth.
Working one at a time, place the top half of a donut down into the glaze. Lift it up and let any excess drip off. Place the donut glazed-side up on the rack and repeat with remaining donuts. Garnish while the donuts are still wet then let sit for a few minutes, until the glaze has hardened a bit.
Try Grant’s Coffee-Flavored-Liqueur and Cocoa, Maple-Whiskey or Spiked Pumpkin Spice glazes for “tipsy” donuts!