Mix and bring to a slow rolling boil. Cook for 3 minutes. Let sauce rest for 5 minutes. Drizzle over cake.

Remove the zest from the limes using a carrot peeler; being careful to avoid the white pith.

Put the zest in the bowl of a food processor fitted with steel blade. Add the sugar, and pulse until the zest is finely minced. Squeeze the limes and measure out ½ cup of lime juice.

Cream the butter in the bowl of an electric mixer fitted with a paddle attachment, then beat in the sugar and lime zest mixture. Add the eggs,one at a time,then add the lime juice and salt. Mix until combined.

Pour the mixture into a 2 quart saucepan and cook over low heat until thickened, about 10 minutes, stirring constantly. The lime curd will be ready when it coats the back of a spoon, and it will register 175 degrees on a candy thermometer. Be careful not to overcook, or it will curdle. Remove from heat, cool and refrigerate till serving.