I don’t know, but I’d imagine it would be better to make vanilla sugar with toasted sugar vs toasting already infused sugar. My reasoning is a lot of what you are probably after in the vanilla is in oils and I suspect they would be diminished after toasting.
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I would start fresh and combine to what you already have? I don't think the previously reacted ingredients would have anything to offer besides burning,
If you have caramel and you re-do the process, you don't get extra caramel-y caramel, you get burnt caramel
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