To make the dough: mix sugar, yeast and soy milk in a large bowl. Allow to rest for 10 minutes.
Then add in flour, salt and oil (40 ml) and knead. until the dough is smooth (the dough should be slightly sticky so don’t add extra flour).
Cover the bowl with a wet towel and allow to rest for 1 hour or until doubled in size.
Knock back the dough and rest again for 45 minutes.
Divide the dough into 6 portions and shape into balls.
Set on a floured cutting board, brush with vegetable oil and allow to rest for at least 1 hour, or until doubled in size.
Heat oil in a large saucepan to 160 - 170°C (320 - 340°F), check the temperature with a jam thermometer. If you don’t have one, dip a toothpick into the oil if it starts steadily bubbling, then it's hot enough for frying. Just be sure that the oil is not extremely hot or the doughnut will fry in the outside and stay raw in the inside.
Fry the doughnuts for 2-3 minutes on each side.
Then transfer to a paper towel to soak up the excess oil.
Once the doughnuts are cool enough to handle, roll them in granulated sugar or powdered sugar.
Spoon the custard or nutella into a pastry tube with a wide nozzle, and poke a hole in the side of each doughnut with it; pipe the filling into the cooled doughnuts.
Serve right away.