Put the water, flour, caster sugar, yeast, eggs and salt into a mixer bowl. Use the paddle attachment to mix on a medium speed for 8 minutes until the dough pulls away from the edges of the bowl and forms a ball.
Leave the mixer on a medium speed and slowly add the butter a little at a time, beating between each incorporation. When all the butter has been added let the mixer run for a further five minutes until you have a smooth and very elastic dough.
Cover the bowl with oiled clingfilm and leave to rise in a warm place for 2 hours – or until the dough has doubled in size.
Punch the air out of the dough then cover with oiled clingfilm. Leave to chill in the fridge for 24 hours.
After 24 hours remove the bowl from the fridge. Use kitchen scales to divide the dough into 15 pieces weighing 60 grams each.
Prepare three baking trays lined with floured baking paper.
Using your hands roll each piece of dough into a ball shape and place on a floured baking tray. Put 5 dough balls onto each tray leaving plenty of space between them. Cover each tray loosely with oiled clingfilm and leave to rise for 4 hours.
Prepare a large bowl filled with caster sugar (for tossing the hot doughnuts in).
Fill a heavy-based pan with sunflower oil. Use a thermometer to bring the oil to 180ºC.
Cut around the kitchen paper so that the dough balls are on individual pieces of paper (this makes it much easier to transfer them). Very carefully peel away the baking paper and place the dough into the hot oil taking care not to deflate it. You can use a floured spatula to lower the dough into the oil.
Depending on how big your pan is, fry in batches of three for two minutes on each side or until golden brown. You may need to use a spatula to hold the dough balls in the oil because they will bob about and float to the surface.
Remove from the pan and drain on a kitchen paper then immediately coat all over with caster sugar. Set aside to cool. Ensure the oil temperature is maintaining 180ºC before you fry the next batch.
Continue until all the doughnuts have been fried and tossed in caster sugar.
Fill a piping bag fitted with a doughnut nozzle (or similar) with the strawberry jam. Pierce the doughnut on the white line in between the fried top and bottom and squeeze the piping bag to fill each doughnut until plump. If you don't have a doughnut nozzle, make a small cut with a pair of scissors in the white line then slide the piping bag in and fill.