Combine the lime zest and sugar. Massage with your fingers for 1-2 minutes to release the lime oils.
Add the remaining the lime juice and yolks, and whisk to combine.
Place over a pot of simmering water and cook, whisking occasionally, until it's thickened considerably and coats the back of a spoon (about 10 minutes).
Remove from heat and strain through a fine mesh sieve. Allow to cool for about 10 minutes.
Cube the butter into 1/2" pieces and add to the curd. Stir until the butter is completely melted.
Optional: Add a drop or two of food coloring to make it green.
Transfer to a jar if you'd like. Place a piece of plastic wrap over the top, pressed against the surface of the curd to keep a skin from forming.
Refrigerate until ready to use.
Notes
Don't use bottled lime juice: The best lime curd is made with freshly squeezed limes. I promise it's worth the effort.
Estimating limes: Depending on their size and how pithy your limes are, you'll need 3-4 limes to make 1/2 cup juice.
Straining: The zest will brown during the cooking process and can appear a little unappetizing. Straining not only removes the zest but produces an ultra-smooth curd.
Coloring: Lime curd is naturally yellow and similar in color to lemon curd. To make your curd green, add a drop or two of green food coloring.
Storage: Lime curd can be refrigerated for up to 7 days or frozen for up to 3 months