Cake flour is HIGHLY recommended for a light texture and loose crumb. Cake flour has a lower protein content than all-purpose flour.
In a bowl, whisk together dry ingredients. In a separate bowl, crem the butter, sugar, eggs, and sour cream together. Stir together with a spoon.
Wrap the dough in plastic wrap, spray the top of the dough with non-stick spray and put in the refrigerator for 1 hour.
Roll the dough out on floured parchment paper until it's ½-inch thickness. Make sure not to over-knead. Cut out with a donut cutter to get about 6-8 donuts.
In a medium saucepan over low heat, add the milk and vanilla extract. Gradually add the powdered sugar and whisk until smooth. You can put it in a water bath to keep warm.
Into a large, deep pan, heat 2 inches of oil to 325 F. Two or three at a time, add the donuts so they do not touch. Fry on each side for 2 minutes.
As soon as the donuts are cool enough to touch, dip them into the glaze and flip to cover the entire donut. Transfer to the cooling rack for about 5 min.