Combine juice, sugar, and eggs in a blender/food processor and blend for about 15 seconds. With the blades running, slowly pour in the melted butter.

Pour the mixture into a saucepan and bring to a low boil over medium heat (this may take a few minutes). Stir frequently.

As soon as it reaches a boil, lower the heat to just above low and stir constantly until the mixture resembles pudding (it should coat the back of your spatula/spoon) and is a little bit shiny. This should only take about two to three minutes after it's boiled. 

Pour into a jar or bowl and let cool a bit before serving or storing.