Add the zest from the lemons into a small bowl. Juice the lemons and measure out 1/4 cup of the juice, then add it to the bowl with the zest (save any leftover juice for another recipe).
Fill a small, high-rimmed saucepan with water approximately 3 inches deep. Bring to a simmer over medium heat.
Meanwhile, in a heatproof bowl, whisk together the yolks and sugar together until smooth. Add the juice and zest mixture, along with the salt. Continue to whisk the mixture until it's well combined.
Once the water is simmering, place the bowl over the pot of water so that the bottom of the bowl is just above the water's surface. Whisk continuously until the mixture is thick enough to coat the back of the spoon. This will take about 8 minutes if using a metal bowl, or up to 20 minutes if using a ceramic or glass bowl.
Once thickened, take the bowl off of the heat. Add the butter, one pat at a time, whisking until it melts into the mixture.
Cover the lemon curd with plastic wrap so that the wrap lightly touches the top of the curd. Refrigerate overnight, or for at least 12 hours.
The next morning (if you can wait that long), take the curd out of the fridge and strain it through a mesh sieve, using a spoon to force the curd through the strainer. Discard the remaining zest in the strainer.
Store the lemon curd in the fridge in an airtight container for up to 2 weeks. Enjoy on top of baked goods, in a tart, or by the spoonful.