Place about 1 inch of water into a heavy-bottomed sauce pan. Heat the water just to a simmer. In a metal bowl, whisk together the sugar and egg yolks until smooth. Place the metal bowl on top of the sauce pan.

Measure out the citrus juice. Whisk the juice into the sugared yolks.

Whisk in the citrus zest.

Continuously whisk the mixture over the simmering water. A layer of foam will form on top of the mixture after whisking for several minutes.

Once the foam begins to incorporate into the mixture, the curd is beginning to thicken.

Once the curd is thick and smooth, whisk in the tabs of butter one at a time. Place the bowl on top of a slightly larger bowl filled with ice to chill the curd rapidly. Lay a sheet of plastic wrap straight down onto the top of the curd to keep a skin from forming. Allow the curd to set in the refrigerator at least 2 hours before serving.