Cut the butter into small cubes. Set aside and let it get to room temperature.
Prepare the double broiler by filling a medium pot with 1 – 2 inches of water. Set a medium heatproof bowl (glass or stainless steel) over. Check the bottom of the bowl to make sure it didn’t touch the water in the pot. Remove the bowl. Bring the water to a full boil over medium heat.
Once the water is boiling, reduce to low heat and keep at a simmer. Add egg yolks, sugar, zests, juices and salt to the bowl. Set it over the pot of simmering water. Whisk constantly until thicken like sour cream, about 10 minutes. Curd will thicken more as it cools. Remove from heat. Add the butter and whisk until smooth.
Transfer the curd to a small glass container or bowl. Cover with plastic wrap by pressing down to the surface of the curd. This will prevent a skin from forming on the curd. Refrigerate until cool.
When ready to serve, remove the plastic wrap and give a few stirs. It’s ready to be used.