Castor (caster) sugar is known as superfine, or bar sugar in the US. It's not the same as powdered sugar which is even finer, has been mechanically pulverized, and is often mixed with starch to prevent clumping.
According to Ochef you can make it with regular granulated sugar in your food processor for two minutes, but let the sugar dust settle before opening.
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Translating cooking terms between US / UK / AU / CA
You didn't heat it enough. "Turning light amber" means that you want caramel. What you did instead was to evaporate water from a sugar syrup, without reaching caramelization temperature. The crystals are not unusual (at some point you get a supersaturated syrup, after enough water has evaporated), but it is better if they don't happen, so don't give them occasion (use a clean pot, don't stir, etc. I suspect the oiling of the pan is meant to reduce nucleation sites).
The recipe is a bit strange anyway, using way too much water. Maybe you can try another one, if this has negative comments. The regular recipes will use some more conventional nut instead of pumpkin seeds, but this doesn't affect the physics of making the caramel. And do yourself a favor and use a candy thermometer. There are experienced cooks who can make sugar candy without one, but if you are not one of them, you'll save yourself lots of errors (and probably enough money in expensive ingredients for the thermometer to pay for itself).
Here is another recipe for brittle. While I haven't tried to make it, I strongly suspect that they know better what they are doing - not at least because they give you the exact temperatures you are aiming for. This is a site with lots of useful tips for candy making, and if you really want to try this without a thermometer, use their chart to learn about the different stages of sugar syrup and caramel.
And choose your recipes carefully. Candymaking (both sugar and chocolate) is even more exact than baking. Directions like "put back on medium for some time" are practically useless. In order to successfully cook a candy recipe, you have to know if not the exact temperature, then at least the desired sugar syrup stage. Maybe somebody who has made lots of brittles could work with a recipe like the foodnetwork one, noticing the problems on the fly and making the appropriate corrections towards the ideal brittle mass. If you don't know what the ideal brittle mass looks like before you start, it is better not to try it that way and go for an exact recipe.
Best Answer
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.