Add the sugar and egg to a small saucepan and whisk together until smooth. Then add the grapefruit juice and zest, and turn the stove on medium heat.

Whisk continuously. The mixture will become frothy as you whisk. When the liquid becomes hot enough to cook the egg, the mixture will begin to thicken and no more liquid will separate from the foam.

It will take at least 5 minutes of whisking over medium heat for the mixture to thicken, but will vary depending on your cookware. The mixture should be thick enough to coat the back of a spoon. Don't be tempted to turn the heat up too high.

Remove the pot from the heat. Add one tablespoon of butter at a time, whisking until each has fully melted into the sauce before adding the next. As the butter is melted into the sauce, it will become less frothy and more silky in texture, and look more glossy rather than foamy.

Once all of the butter has been whisked in, transfer the curd to an air-tight container and refrigerate until completely cooled. The curd will thicken further upon cooling.