In a medium bowl, combine the flour, sugar, yeast and salt.
Make a well in the centre and add warm milk, butter and egg yolks. Mix with a wooden spoon bringing the dough together before turning it out onto a well floured surface. Knead until smooth for approx. 5 minutes. This can also be done in electric mixer fitted with a dough hook.
Place dough in a greased bowl, cover with cling film or a clean cloth and place in a warm spot until dough has doubled in size for approx. 90 mins.
Turn dough out onto a floured surface, punch down and knead for 2 minutes until smooth. With a floured rolling pin, roll dough out to 10mm thickness.
Use 2 round cutters (8cm and 3.5cm) to cut out doughnut rounds and centres. Place on a baking sheet lined with baking paper and allow to rise slightly for another 15 minutes.
Prepare cinnamon sugar coating by mixing the two ingredients together in a medium size bowl and set aside. Heat oil to 180°C if using a deep fryer or until a small piece of dough starts to sizzle if using a pot on the stovetop. Deep fry doughnuts for 45 seconds each side or until puffed and golden. Deep fry the donut holes too, these will cook a little quicker so turn them to avoid overcooking the holes.
Immediately toss doughnuts and holes directly into the cinnamon sugar to coat both sides. Repeat with remaining doughnuts and holes.