How to use lemon or other fruit acid in Brigadeiro without curdling?
As someone pointed out, key lime pie (or "torta de limão com merengue" as we know it) shouldn't curdle, but it is a different type of heat we're talking about, so bear with me for a while.

A few tips:

1) Add less zest at the end of the cooking. If you add everything together, the essential oils evaporate during cooking and you lose the flavor;
2) If you want to use juice, make a white brigadeiro and add the lime juice (our "limão taiti" is actually called lime) after cooking so it doesn't curdle and preserves the flavor