Combine all the ingredients in a heat-proof bowl and set it over a bowl of simmering water, making sure the bottom of the bowl doesn't touch the water.
Stirring continuously, allow the butter to melt and the mixture to come together. At first the mixture is going to look strange and curdled but just continue stirring and when the butter is melted, switch to a whisk and whisk until the curd is thick and glossy.
Taste to ensure the curd is smooth and cooked through then remove from the heat.
Transfer the curd to clean jars and keep in the fridge for up to 2-3 weeks.