At least 1 hour before serving
Preheat the water bath to 180°F (82°C).
Sterilize a 1-pint glass canning jar, lid, and ring; set aside.
Place the eggs and sugar in a mixing bowl and beat with a hand mixer until the sugar dissolves and the mixture thickens slightly, about 2 minutes. Whisk in the lemon juice and melted butter. Do not overbeat or the mixture will be frothy. Pour the curd into the prepared jar, filling to the brim. Seal the jar and immerse in the water oven.
Cook for 45 minutes, then remove the jar from the water. Open the jar lid and stir the curd with a spoon for 1 minute. The curd loses some volume as it thickens, so the cooked quantity is about 1¼ cups. Place the open jar in a bowl of ice and water and let cool. To store, reseal the jar and refrigerate for up to 2 weeks.
Iced Lemon Curd Mousse
Serve this refreshing iced dessert in small glasses to show off the pretty color of the swirled lemon curd and cream. Although they're perfectly delicious served cold from the refrigerator, I like to freeze the mousses for about an hour before I serve them. The mixture doesn't harden, but it does get ice-cold. If you have lemon curd on hand, this makes a quick and easy yet elegant finish to a meal.