Bring a large pot of water to a simmer. Place a large bowl on top, it shouldn’t touch the water. Whisk together the sugar, lemon juice, lemon zest, egg yolks, egg, and salt in the double boiler over medium heat. Whisk constantly for 12-15 minutes until the curd begins to foam and then thicken. Once thickened, remove the bowl from the heat and stir in the butter a little at a time until shiny.