Beat ingredients B in low speed about 1 minute until sandy.
Add egg yolk and beat in medium speed until light coloured and thick.
Alternately add ingredients A and the sour cream in three separate addition, starting and finishing with the dry ingredients. Mix in low until all combined.
Cover and put in the fridge for 45 minutes to 24 hours.
Flour the work surface then roll out the dough about 8 inch diameter.
Cut with doughnut cutter.
Fry them in medium heat about 325F. Do not overcrowd the frying pan. Once the doughnuts float, fry for 15 seconds, then gently flip them. Fry for 75 to 90 seconds, until golden brown and cracked, then flip and fry the first side again for 60 to 75 seconds, until golden brown. Transfer to a rack set over paper towels.
Glaze : Mix all the ingredients until smooth.
To glaze, dip one side of each hot doughnut into the warm glaze, and let dry for 10 to 15 seconds before serving.
