2 oz passion fruit juice
1 tsp sugar
Preparation: Combine all non-sparkly ingredients and stir. Add the cava, very slowly otherwise it will react with the sugar and overflow. Add quarter of a passion fruit as garnish and serve.
General Comments: I think really this should use passion fruit syrup rather than juice plus sugar, but the final result is pretty similar, just a bit less thick and a lot easier to source. Whether you want to source it is another matter, of course; as is so often the case with cocktails containing cachaca, the best thing you can really say about its inclusion here is that it doesn't do too much damage to the taste. Cachaca is even worse than vodka in this regard; you don't add it to drinks to improve them, you do it to get rid of cachaca you've somehow ended up with if you find the smell too unpleasant when using it to scour your floors. 
Still, the passion fruit is nice (though the chunk floating towards your face each time to take a drink is a little disconcerting), and its low-level tartness is offset by the sugar. Bubbles, as usual, are fun and ticklish, and the whole thing is pretty cheap. Really, though, there's nothing here to recommend this over other sparking white-based cocktails, especially since it's a little more hassle than most.
 I should really note here that we only have cachaca because some friends of ours were ludicrously generous and brought a bottle of it to one of our cocktail parties (which are rather more free-form and shouty than that phrase typically implies: dress code is casual/vomit-stained). They weren't to know I would take so utterly against it, and we remain grateful for any and all additions to our booze supply AKA the Magic Cupboard.