Dice the potatoes into 1cm cubes as evenly as possible.
Heat the oil in pan, fry the onions to soften. Add the curry leaves and curry powder. Mix well.
Add the potatoes and fry for 1 minute. Add water, sugar, salt, and soya sauce. Cook until the potatoes are soft. Taste. Adjust seasoning if necessary.
Cool the mixture and transfer to the fridge.
When you're ready to cook the puffs, preheat the oven to 180C/350F/gas mark 4 and take the pastry out of the fridge (this should be at least 15 minutes before using).
Dust a surface with plain flour. Roll out the pastry and fold into 4 vertical equal sheets. Using a knife or pizza roller if you have one, cut along the lines.
Fold one vertical sheet into 3 squares. Repeat on the other three. You should have 12 squares.
Place one square in front of you in a diamond shape. Egg wash the edges with a brush.
In each square of pastry, place a couple of cheese triangles (with the point facing down) and a spoonful of potato mixture above. Fold the pastry up into a triangle. Pinch the edges together to seal, then crimp with a fork along the edges. You can freeze the puffs at this point to enjoy later.
Brush the pastry with the beaten egg and sprinkle over the nigella seeds. Bake for 15-20 minutes, until golden.
Serving Size 4