Eggs: 3 whole eggs (or 2 eggs + 2 yolks for extra richness)
Sugar: ¾–1 cup, depending on tartness desired
Butter: ½ cup (1 stick), cut into small cubes
Juice: ½ cup fresh lime juice (about 4–6 limes)
Zest: 1–2 teaspoons finely grated lime zest
Pinch of sea salt
Combine sugar, zests, and eggs in a heavy saucepan. Whisk until smooth.
Add juice and salt, then place over medium-low heat.
Stir constantly with a heatproof spatula or whisk until thick enough to coat the back of a spoon (about 170°F / 77°C).
Remove from heat, whisk in butter a few pieces at a time until fully incorporated and glossy.
Strain through a fine sieve to remove zest or any curdled egg.
Cool to room temperature, then refrigerate. It will thicken as it chills.