In a saucepan that a bowl can sit on top of, pour about 2 inches of water and bring it to a simmer. (The bowl should be able to sit on top without touching the water)
In a bowl, add lemon juice, eggs, yolk, sugar, and salt. Place the bowl on the saucepan and whisk the curd continually until the mixture for 10-12 minutes or becomes very thick (or it is 180° F on a thermometer.)
Remove the bowl from the saucepan and cool until it reaches 140° F, stirring from time to time to release the heat.
While it is cooling, cut butter into 1-tablespoon cubes. When the cream is ready, whisk in the butter 1 tablespoon at a time, stirring until melted after each piece.
Once all the butter is added, the curd will be pale yellow and thick.