Combine the dairy-free milk, water, melted margarine, sugar, flour, salt and yeast in a food mixer fitted with a dough hook.
Mix for 10-12 minutes until the dough has come together in an even, slightly stick ball. Turn out onto a lightly floured work surface and allow to rest for 10 minutes.
Roll the dough out to 1 cm in thickness and cut out rounds using a 4-5 cm straight-sided cookie cutter. Arrange the rounds of dough on lightly oiled trays.
Cover loosely with clingfilm and leave to rise in a warm place until doubled in size; usually 30-40 minutes.
When ready to cook the doughnuts, heat the oil to 180°C/350°F in a heavy-based saucepan.
Fry in batches until the doughnuts are golden brown in colour, flipping over to ensure they are evenly coloured; usually 2-3 minutes. Remove with a slotted spoon and drain on kitchen paper.
Arrange on a serving plate and dust with the remaining caster sugar.
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