Whisk together in a medium bowl: flour, baking powder, baking soda, salt and cinnamon.
In a large bowl, beat eggs until foamy.
Gradually add and beat until thoroughly blended: sugar, sour cream and vanilla.
Add the dry ingredients and stir just until incorporated. The dough will be very soft. Pat the dough into a disk, wrap it in plastic, and refrigerate for at least 2 hours or up to 2 days. The dough will never become firm but it will be workable when cold.
Working on a lightly floured surface, pat or roll the dough out 1/2 inch thick. Cut with a well-floured doughnut cutter, keeping the doughnut holes too. Drop the doughnuts and holes, 2 or 3 at a time, into oil heated to 365°F, Fry until golden on both sides. Drain well on paper towels.
shake in a bag with: cinnamon and sugar
Serve while still warm or within a few hours of frying.