A thermometer is particularly helpful to ensure the oil is the right temperature – not too hot or not hot enough. Alternatively just use a small piece of dough to test with.
Combine yeast, milk and sugar in a small bowl and whisk. Let stand for 10 minutes until mixture has activated and the mixture is foamy.
Place the flour in the bowl of a stand mixer with dough hook, make well in the centre and add the yeast mixture, yolks and butter. Knead on medium speed for about 5 minutes, until you have a smooth, pliable and soft dough.
Form a disc shape, place in an oiled bowl and cover with plastic. Set aside in a warm place for 1 1/2 hours or until doubled in size.
Lightly dust work surface with flour and roll out the dough to about 1.5cm thick. Using a cookie cutter approximately 5cm, cut out rounds of dough. Re-roll any remaining dough and continue until all used.
Place on a piece of baking paper, cover with a tea towel and let rise for 30 minutes.
Line a shallow plate with baking paper to drain the donuts and in another shallow plate or bowl combine the caster sugar and cinnamon sugar for rolling.
Pour canola oil into a large saucepan and and place a thermometer on the side. Turn heat up as high as possible. You want the temperate to be approx 180C.
Drop donuts in the hot oil 3 at a time and cook for about 20 seconds on each side. Remove from pan with slotted spoon and let drain for a few seconds.
Place in the bowl of cinnamon sugar and dust on all sides then transfer to a serving plate.
Variation; if you prefer to bake your donuts, prepare mixture as above but cut into rounds or use a donut and place on a baking tray and bake in 180C oven for approximately 15 minutes or until risen and golden. As soon as they come out , dip baked donuts into a little melted butter then the mixture of cinnamon and sugar