Heat the egg/juice mixture over medium low heat, stirring constantly. You must be sure to keep stirring across the entire bottom and corners of the pot so the custard does not have a chance to stick. The mixture will start out opaque with a fine white foam over the surface. As it heats up the curd will thicken, become more translucent and the foam will disappear.

Cook until the curd will coat the back of a wooden spoon and just begins to boil. Don't allow it to come to a rolling boil. One or two bubbles is all you need to see.

Immediately remove from the heat and pour through the sieve over the butter and gelatin. You can skip straining the curd if you like the bits of zest in the curd.

Stir until the butter and gelatin are melted and thoroughly incorporated. Cover the bowl with plastic wrap and cool a bit before putting in the refrigerator.

Refrigerate until completely cooled. If using gelatin, allow it to sit 8 hours or overnight to gel.