In India, we use fine sugar known as "bura chini (or bura sugar)". Recently, I read a recipe for scones which required caster sugar. While, I am not quite sure it they are same but both have quick dissolving properties and are superfine. I am curious if one can be substituted for another.
Caster sugar is just ordinary table sugar that has been ground to a finer texture. It is still grainy like sand rather than completely powdered.
Bura, on the other hand, is caramelized and dehydrated. It is typically made by dissolving table sugar in water, then boiling off the water. This results in a texture like that of caster sugar, but a deeper flavor.
Generally sugar in a recipe provides more than sweetness. The texture of the end result is affected by the sugar. Since bura has less water than regular caster sugar, it will probably result in a drier scone.
Also, part of the taste of any baked good comes from the caramelization of sugar that occurs during the baking. Since bura is already caramelized, it will be more prone to burn.
Finally, bura is intended for desserts like laddus, where sugar makes up a much higher proportion of the ingredients and the overall flavor is supposed to be sweet. Why use it for something like a scone? Sweetness is an important contributor but not the main goal for scones, and there is no point using something as complex as bura when its flavor profile will be overwhelmed by the other ingredients.
For these reasons, I think you are better off just grinding ordinary table sugar rather than using bura. If you don't want to go through the bother of grinding, I think you'll have better luck using the sugar as is instead of using bura; though that isn't a great choice either.
Well, in India its a bit hard to find castor sugar. I usually make a powder of granulated sugar that is commonly available in the supermarket. I think it makes a great substitute, and I have used it many times for baking. And if required you may even pass through a fine sieve, in rare cases though. Hope this helps!
The most direct answer is "Yes, you can use bura in place of castor." with the caveat...
... that you may need to adjust to quantity to get the desired taste
... you may need to process the sugar through a food processor or spice grinder if the consistency isn't what you want.
In any case the result should be close enough to tweak it until it comes out just how you want and still be able to enjoy the 'mistakes'.