Combine flour, baking powder, baking soda, salt and cinnamon together in a bowl. Set aside.
In a mixer beat eggs until light and fluffy.
Mix in sugar.
Add sour cream and vanilla. Stir until combined.
With mixer running, add flour mixture. Mix just until ingredients combined.
Use a spatula to scrape bottom and sides and fold in any unincorporated ingredients. Dough will be sticky and wet.
Place dough on a piece of plastic wrap. Wrap up sides, form dough into a disc and place in fridge for 1-2 hours.
Forming Doughnuts
Remove dough from fridge. It will not be firm and will still be sticky.
Place dough on a well floured surface. Sprinkle with more flour on top. Knead dough once or twice to incorporate the flour into the dough.
Add a bit more surface and to the top of the dough. Gently press dough down with fingertips into a rectangular shape. Dough should be approximately ½" thick.
Using a doughnut cutter dipped into flour, cut out shapes. Dip doughnut cutter into flour before each cut.
Fill fryer or pot with at least 2 inches of oil. Heat to 375 degrees.
Add 2-3 doughnuts at a time. Cook 2-3 minutes per side until golden brown. Remove from oil onto a baking rack.
Repeat until all doughnuts are cooked, reheating to 375 degrees between each batch.
Cinnamon and Sugar
Mix cinnamon and sugar together in a bowl.
When doughnuts are almost cooled dip each doughnut into mixed cinnamon and sugar.
Notes
Tips for Making Cake Doughnuts
Combine all ingredients to make a dough
Place dough on plastic wrap and refrigerate for 2-4 hours
Shape dough into a large rectangle
Cut with round cutter
Fry doughnuts in a preheated 375 degree oil
Only fry 2-3 at a time so oil is not overcrowded
Place cooked doughnuts on a baking rack so excess oil can drain