Put the caster sugar, rice vinegar and soy sauce in a saucepan and simmer for 2-3 minutes until the sugar has dissolved.
Add the tamarind paste and ketchup, stir and remove from the heat.
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Notes
If your sauce is a little too runny, keep it bubbling for a further minute or two until thickened.For a spicy amai sauce, add a squeeze of sriracha or a pinch of chilli flakes.