Preheat the oven to 180ºC (350ºF).
CARAMEL topping: Bring to a boil the sugar and water with lemon or lime juice in a saucepan, stirring constantly over medium heat until the sugar melted. Stir continously until the syrup thickens and turns golden brown. Pour the syrup into custard cups or small molds.
EGG Custard: Combine the sugar and water in a saucepan over medium heat, stirring constantly until the syrup thickens. Let it cool. Add the butter and egg yolks, beating well and mixing thoroughly. Pour gently the mixture into the caramel-lined cups. Cover each cups with aluminum foil and place in a "bain-marie". Bake for 1 hour or until the mixture is firm and until a skewer/toothpick inserted into the center comes out clean. Remove from the oven and allow to cool.
Serve well chilled. Share and enjoy the deliciousness!
