2 tablespoons unsalted butter, cut into small cubes and chilled.
Whisk together the sugar, lime zest, and lime juice in a heavy medium saucepan. Whisk in the yolks. Drop in the butter. Cook over medium-low heat, stirring continuously with a heatproof spatula until the mixture is thick, smooth, and beginning to form small bubbles just around the edge of the pan, about 8 minutes.
Remove from the heat and pour through a fine sieve into a small glass bowl. Press a piece of plastic wrap directly onto the surface of the curd to prevent a skin from forming. Refrigerate until chilled, at least 4 hours.
What else works? You can use this recipe to make lemon curd or orange curd by changing the juice and zest.
