Yes you can. Boom//Bust has a converted mana cost of 2 and 6, so it will satisfy the condition for the Elf's cascade trigger. Then you can play either half without paying the mana cost, so you can choose to cast Bust if you want.
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DCI Level 2 Judge
Scientists have calculated that the chance of anything so patently absurd actually existing are millions to one. But magicians have calculated that million-to-one chances crop up nine times out of ten.
Yes you can. It says this right on Bloodbraid's rulings on the gatherer page:
If you exile a split card with Cascade, check if at least one half of that split card has a converted mana cost that's less than the converted mana cost of the spell with cascade. If so, you can cast either half of that split card.
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Standard: RBRbDWBR WUU/W ControlUW
EDH: WGRMayael the AnimaRGW WBTeysa, Orzhov ScionBW
(Sorry i don't know how to card tag split cards)
Scientists have calculated that the chance of anything so patently absurd actually existing are millions to one. But magicians have calculated that million-to-one chances crop up nine times out of ten.
RBRbDWBR
WUU/W ControlUW
EDH:
WGRMayael the AnimaRGW
WBTeysa, Orzhov ScionBW
My Trade Thread:
http://forums.mtgsalvation.com/showthread.php?t=254000