Can I substitute the icing sugar with caster sugar in Nigella's Nutella Cheesecake? Will it make a difference?
Icing (confectioners') sugar is a fine powder whereas caster (superfine) sugar has very fine granules. Icing sugar dissolves very easily and in the Nutella Cheesecake (from Nigellissima) will give a very smooth texture. You could possibly try beating caster sugar with the cream cheese for a little extra time to help the sugar dissolve, but we suspect that you would not be able to get the mixture completely smooth and so the cheesecake itself would have a slightly grainy texture as it is refrigerated and not baked. In a baked cheesecake the heat of the oven will help caster sugar to dissolve.
So we would not recommend switching to caster sugar.
If you have a clean coffee grinder then you could probably grind some caster sugar to the powdery texture of icing sugar and use that. You may need to grind a small amount of sugar first and discard it, to make sure that the coffee grinder is clean. You can usually use a food processor to grind granulated sugar into caster sugar, but it can be difficult to grind it beyond that.
By weight you would use the same weight of caster sugar and icing sugar. But if you are measuring by volume, using cups, the measurements for caster sugar and icing sugar will be different