Mysterious, beautiful fruit

This is a beautiful fruit that we found in Nicaragua. I think it is a sort of cactus flower. It looks like an artichoke from the outside. Once you cut it open it is full of a very subtle, sweet fruit, with very small seeds. The first time I saw it was in an ice cream flavor. Then we began drinking it all the time in cafes. A little lemon or lime juice is added to give it more flavor.
Here's the drink!