Stir together the flour, caster sugar and salt in your bowl.
Rub the butter into the flour so that no large lumps of butter are left
Make a well in the middle of the flour and add the egg, milk and water.
Mix to form a dough using the dough hook on your stand mixer or a wooden spoon if mixing by hand.
In the stand mixer, knead the dough on a low speed for 8 minutes
Or if mixing by hand, tip the dough out onto a lightly floured boeard and knead by hand for 10 minutes.
Place the kneaded dough back into the bowl, cover and leave to rise for 1 hour or until doubled in size.
When the dough has risen, tip it out onto the lightly floured board and knead gently to break down any large air bubbles that have formed.
Divide the dough into 9 or 10 pieces and form each piece into a ball.
Place the balls of dough onto well greased baking trays, cover with oiled cling film and leave to rise for another hour.
When the dough has risen, heat up you oil in a deep fat fryer until it reaches 170°c.
Use a wide spatula to lift the first doughnut carefully so as not to deflate it, and place carefully in the hot oil
Depending on the size of your fryer you should be able to fry 3 or 4 doughnuts at a time but don't crowd them in as they will cool the temperature and they will then soak up the oil.
When one side is nice and golden, flip the doughnuts over and fry the other side.
Carefully lift out the golden doughnuts and place on crumpled kitchen roll to drain while you put the next batch in the oil.
Put one doughnut at a time into the extra caster sugar, turning to coat it all over.
Place on a wire rack to finish cooling and continue with the other doughnuts until they are all done.