In a heat proof bowl whisk together the egg yolks, sugar, key lime zest, key lime juice, and salt.
In a medium pan place 2-3 inches of water. Bring to a boil then reduce heat to a simmer.
Place the bowl over top of the pot creating a double boiler.
Cook over medium low heat, whisking constantly, until the curd has thickened.
Remove from heat and stir in the butter until smooth.
Place in a jar and put in the refrigerator for up to 3 weeks.