Whisk together warm milk (make sure it isn't too hot or it'll stop the yeast from working) with the melted butter and egg.
Stir in the yeast, 1/4 of a cup of sugar (save the rest for rolling the fried doughnuts in) and the flour. Mix together then move onto a floured board and knead until you can feel the texture of the dough begin to change and become smooth, springy to the touch and slightly elastic.
Place back in the bowl and cover with clingfilm. Set aside for an hour or so until the dough doubles in size.
Roll out the dough to roughly 1cm thickness and use a 4cm cookie cutter to get roughly 25 circles of dough.
Pat or roll each circle into a disk and put around half a teaspoon of strawberry jam in the centre. Carefully fold the dough circle in half with the jam in the centre then pinch the edges together to make a semicircle. Fold the outer edges in and then the top to create a small ball of dough. Roll in your hands to smooth.
Heat the oil to 170°C/340°F and fry the doughnuts in batches until golden brown and puffed up. When they are still warm roll in the leftover caster sugar and serve.