In a mixing bowl, combine the sugar, salt and flour. Set this aside.
Place the water and butter into a saucepan. Slowly bring this to the boil, melting the butter along the way.
Work quickly - take the pan off the heat, add the flour mixture, and use a wooden spoon to stir vigorously to combine.
Return the pan to the heat and cook - still stirring - for about 1 minute or until a smooth, thick mass forms. It will come away from the sides of the pan when ready.
Transfer the mixture from the pan to a mixing bowl. Set it aside for about 5 minutes to cool slightly.
Gradually add the egg. Beat the mixture constantly to mix the egg through and make sure the mixture is completely combined before each new addition.
Keep beating and adding egg until the mixture becomes glossy and falls heavily from the beaters. This may not take all of the egg - when the mixture is ready, it should be able to fold in on itself when cut through with a knife or spatula.
With the final addition of egg, add the lemon zest and `mix it through.
Heat the oil in a medium saucepan to 180C - at this temperature, if you drop in a cube of bread it should turn golden brown in about 15 seconds.
Drop a heaped teaspoon of the batter at a time into the oil. Do this for 5 or 6 drops, so that you can cook in batches of 5 or 6 beignets.
Cook for about 4 minutes, turning after 2 minutes, or until golden and cooked through.
Use a slotted spoon to remove the beignets from the oil. Place them onto some paper towel to drain excess oil.