Sift the flour and salt into the mixing bowl of a food processor, then mix in the sugar and the yeast.
Beat the milk and eggs with a fork, then make a well in the centre of the flour mixture and add the liquid.
Mix the dry ingredients in from the sides using a wooden spoon.
Blend the mixture at a slow speed for ten minutes using the dough hook of the food processor.
It will become quite sticky. Cover with a damp dishcloth and leave in a warm area for approximately ninety minutes or until the dough has doubled in size.
Knock back the dough using your hands, then squeeze the soft butter into the dough, ensuring that it is thoroughly mixed.
Cover and refrigerate for at least four hours or overnight, if possible.
Remove the dough from the refrigerator and knead for a few minutes.
Roll out onto a lightly floured surface until it is about 2,5cm thick, then use a scone cutter to cut out rounds.
Use a smaller cutter to make a hole in the middle of each round to form a ring.
Repeat with the remaining dough. Place the rings onto a greased baking tray, leaving space for rising.
Leave to rise at room temperature for thirty to fourtyfive minutes or until they have doubled in size.
Heat the oil in a deep-fryer or a medium-sized saucepan until it measures 180 degrees.
Use an oil thermometer to test whether the oil is ready, or drop a cube of white bread into the oil.
It should turn golden brown within fourty seconds. Fry two to four doughnuts at a time, until golden and cooked through.
This should take approximately five minutes. Remove the doughnuts from the oil and drain on absorbent paper.
Repeat with the remaining doughnuts.
Place the doughnuts onto cooling racks and dust with icing sugar.