Combine all dough ingredients in a large bowl, reserving 32.5ml of water.
Stir the mixture by hand until a dough forms.
Add the remaining water and knead in the bowl for four minutes.
Knead the dough on a floured surface for 10 minutes until smooth.
Place dough in a clean bowl, cover with a damp towel, and rise for one hour.
Knock back the dough by kneading it briefly on a floured surface.
Divide the dough into 10 equal portions and shape them into balls.
Place balls on a floured tray and rise for one hour.
Preheat a deep fat fryer with sunflower oil to 180°C.
Fry each doughnut for five minutes per side until golden brown.
Remove from oil and immediately roll in caster sugar.
Allow the doughnuts to cool completely.
Make a small cut into the side of each doughnut reaching the center.
Fill a piping bag with strawberry jam and pipe generously into the doughnuts.