Place the warm water in a small bowl, add a sprinkle of sugar and stir in the dried yeast.
Sift the flour, sugar and salt into a large mixing bowl. Add in the cold butter diced into small cubes and rub into the flour until you form a bread-crumb like consistency.
Add the yeast mixture and the eggs into the flour mixture, and knead together until a soft dough.
Knead the dough for 10-15 minutes until it forms a soft elastic dough.
Put the dough in a clean bowl and seal with clingfilm. Place in a warm place and allow to prove until it doubles in size.
After it has doubled in size, knock the dough back and divide into 8 pieces.
Roll the pieces into balls on a floured surface and shape it into a dough by apply a little pressure on top.
Place on a baking tray, and allow to prove for another hour.
Fill a large bottom pan half full with the oil. Heat the oil to 180°C.
Place the caster sugar in a bowl ready to toss the doughnut onced cooked.
Place the doughnut into the oil and cook for a couple of minutes on each side until golden brown.
Once, cooked transfer on to kitchen roll before rolling in the sugar.
Make a small hole in the doughnut with the piping nozzle and fill the doughnut to the brim with the custard.
Serve and enjoy!