Add salt, yeast, sugar and flour to a bowl. Mix in butter, milk and egg yolks until a dough forms.
Kneed on a floured surface until smooth and place in a large greased bowl. Cover with a tea towel and leave to prove until doubled in size (usually 1.5 hours)
Kneed the dough again on a floured surface for 2 minutes. Use a roller to roll the dough flat to roughly 1cm. Use a round cutter to cut the doughnuts to size and remove the centres. Leave to rise for 20 minutes.
On a plate, combine 1 cup caster sugar and cinnamon. Heat a pot of canola oil to deep fry.