In a bowl press together the butter and sugar until crumbly. Add the egg yolks and stir until well combined.
Sift the flour, baking powder and salt into a separate bowl.
Add to the sugar egg mixture 1/3 of the flour and half the sour cream. When well combined add another 1/3 of the flour and 1/2 the sour cream. Finally mix in the remain 1/3 flour. Place in the fridge while you prepare the bench.
Lightly flour the bench then roll-out the dough about 1cm thick. Cut large circles, then a small circle out the centre to create a doughnut shape.
Preheat your air fryer to 180 degrees C.
Using the melted butter and a brush, place a thin layer on both sides of the doughnut then add to the air fryer. Cook for 8 minutes.
As soon as the doughnuts are cooked, paint again with the melted butter and immediately dip into the cinnamon sugar mixture.
Serve and eat hot. Like traditional doughnuts they are much better hot than cold.
Cook time may vary on strength and capacity of your air fryer, check and adjust accordingly.