Once all of the key lime and egg mixture is fully combined add it all back to the pan and place over medium heat, stirring continuously with a wooden spoon. Cook for 2-3 minutes
Take off the heat and immediately stir in butter cubes to bring the temperature down. Once butter is fully combined add heavy cream, vanilla extract, and salt.
Pour into a container and cover with plastic wrap that is directly on top of the key lime curd. You want it fully covered so skin does not form on top.