Zest the grapefruit using a microplane, then juice the grapefruits until you have one cup of juice.
Simmer the grapefruit juice over medium heat until reduced by half.
Cream the butter and sugar in a metal bowl. Add in the egg yolks and eggs one at a time, whisking after each one.
Stir in the zest and pinch of salt, then slowly add in the grapefruit juice and lemon juice, stirring as you go.
Fill a saucepan with a few inches of water and bring to a simmer over medium heat. Place the metal bowl on top of the saucepan for a makeshift double boiler. Stir the curd constantly so it doesn't curdle.
Continue to heat the curd and stir frequently for about ten minutes. It should thicken as it cooks.
Once it has thickened enough to coat the back of a spoon, it's done. It will continue to thicken as it cools. Pour into a pint jar and refrigerate.
You may substitute limes or Meyer lemons for the grapefruit.