Whisk eggs, yolks and salt together in a metal bowl.
In a heavy-bottomed sauce pan, combine juice and sugar. Stir and heat to just below boiling.
Temper into eggs and pour everything back into the pot.
Heat over medium, whisking constantly and fairly quickly until thickened (temp will be about 160F, if you're using a thermometer). As you are whisking, the curd will get very foamy on top. As it heats and thickens, the foam will diminish. When the foam is completely gone, this is a good signal that it's done and you can stop whisking and immediately strain into a bowl and stir in butter until melted and combined.
Depending on how thick and buttery you want your curd to be, use more or less butter. More butter will yield a firmer curd when chilled, but the texture will be silkier if you use less butter.
Put plastic wrap directly on curd and chill until you are ready to use it. Or, for a pie or tart, pour directly into your pre-baked pie or tart shell and let it set up in the fridge.