In a bowl combine the butter and sugar until crumbly. Add the egg yolks and stir until smooth.
Sift the flour, baking powder and salt into a separate bowl. Add 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 remaining flour.
Lightly flour the bench then roll the dough until about 1cm thick. Cut small circles about the size of a doughnut hole, or don't roll flat, just make equal sized balls.
Preheat your air fryer to 180 degrees C.
Using the melted butter and a brush paint the balls then place in the air fryer. Cook for 6 minutes.
As soon as the doughnut centres are cooked, paint again with the melted butter and immediately dip into the cinnamon sugar mixture.