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Cook all the remaining ingredients (except the butter and the gelatine if you're going to use it) on a low heat. Keep an eye on the exact temperature - the mix should never exceed 85°C (185F) or the eggs might cook.
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When the mix starts to thicken, get ready to remove the pan from the heat. How do you know when it's ready? Use the whisk to create a whirl - if the whirl continues to spin for a bit before stopping, you will need to keep it on the heat for a bit longer. If on the other hand it stops immediately, it's ready - you can stop cooking and remove from the heat.
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If you want to add gelatine to the mix, do so now.
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Strain the lime curd to remove the zests and the basil leaves.
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Pour the lime curd into a beaker and let it cool until it reaches about 40-45°C (105-115F). This should take about 5 minutes.
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Using a hand-held blender, start mixing the curd at a low speed, then start dropping in the butter and continue to mix until smooth.