In a non-reactive saucepan bring lime juice and 1/2 of sugar to a boil.
In a separate bowl whisk eggs for two minutes and quickly whisk in remaining sugar. Pour 1/2 of boiling lime syrup into eggs and beat well. Pour egg mixture into remaining lime syrup and whisk back to a boil. Boil for one minute while constantly whisking and mixture has the consistency of thick bubbling honey.
Remove from heat and cover with plastic food wrap.
Mix gelatin with water and set aside for five minutes.
Add gelatin and stir into above. Once incorporated add SOFT butter in small nuggets and stir with a spatula. If butter was too cold and refuses to dissolve in curd, place in a microwave for a few seconds. Let cool in refrigerator.