In a bowl, sift together the cake flour, baking powder, salt.
In the bowl beat the butter and sugar together until sandy. Add the egg yolks and mix until light and thick. Add the dry ingredients to the mixing bowl in 3 additions, alternating with the sour cream. The dough will be sticky. Cover with plastic wrap and chill for 1 hour.
On a floured surface, roll out the dough to about 1/2 inch thickness. Use a doughnut cutter or two differently sized biscuit cutters to cut out as many donuts as possible, dipping the cutters into flour as necessary to prevent sticking.
Fry the doughnuts a few at a time, being careful not to overcrowd the pot. Fry on each side about 2 minutes, being careful not to let them burn.
For Chocolate Glaze
Sift together the sugar and cocoa powder in a medium bowl. Slowly stir in the milk and vanilla, a little at a time, to make a smooth, pourable glaze.
Then dip the doughnuts in glaze and keep in for 2 minutes and serve..