With the magic of google: "This is the British term for sugar with small grains that are between granulated and icing sugar in terms of fineness. It is sometimes spelled castor sugar, and is known as 'superfine' sugar in America"

I think it's powdered sugar

It's orginally a post from the French version that got (poorly, according to your comment) translated into English

I Google caster sugar, it's very fine granulated sugar

Their comment made sense though. castor sugar is more fine than granulated sugar and less fine than icing sugar. it is actually icing sugar that is powdered sugar. I think. at least it was described as practically a powder in fineness. The closest thing the us has is extra fine granulated sugar and/or baker's sugar though it does appear sugar that is castor quality is harder to find in the US than places like the US. Don't quote me on that though. My source was actually multiple sourced via a comment conversation and there is a chance they were all wrong ...

And they can all be pretty much answered with one word: Teenager.

I thought #2's comment was bad, but then I saw yours

Thank you, I'll be here all week.

Powdered sugar and castor sugar are different. Castor is very fine grains of sugar, not mixed with anything.

The best way to make Caster sugar from granulated sugar is to grind it up in a pescle and mortar ^^

In Belgium, due to their complicated past with the Congo and the rubbersugar harvested there, any confectionery is a sensitive topic. You can actually be fined upwards of 300 Belgian Wapfeln (approx 15 USD) for throwing away half a sugar pack at a coffeeshop.

Putting sugar in the microwave probably leads to burn caramel all over the place. I'm not positive, I've never tried it, but it was probably an enormous, stinking, glue-like mess.