Add sugar, butter and egg yolks to a large bowl then cream until pale yellow.

Add sour cream and mix until combined.

Mix in baking powder and salt.

Into another mixing bowl, add all-purpose flour and corn flour, whisking to combine well. 

Add flour mixture into the creamed mixture, about a cup at a time, mixing between each addition, until combined and no flour streaks remain. Use your hands to bring the dough together. 

Transfer dough onto plastic wrap and shape the dough into a rectangle, so that it’s easier to shape later. 

Place in the refrigerator for at least 3 hours (or up to 3 days).

After at least 3 hours, remove the dough from the refrigerator.

Dust the table with some flour and roll out the dough with a rolling pin, flipping the dough over each time.

Roll the dough till it is an inch thin.

Using 2 different sized round cookie cutters, cut out the donut shapes and donut holes.

Add oil to a heavy bottomed pot and heat over medium heat until very hot.

Carefully drop the donuts, a few at a time into the hot oil. Fry about 2-3 minutes each side or until golden brown. Transfer the cooked donuts onto a wire rack, letting the excess oil drip and allow to cool.

When frying donut holes, they'll only need to fry about a minute per side. Transfer to rack and cool.

Make sure the oil is not too hot, if not the dough gets burnt on the outside but does not fully cook on the inside.