I went to Z-Burger a bit ago and really enjoyed it, and they have 75 flavors of milkshake. I tried the mango kiwi one last time, and it was awesome.
To get said shake, all you have to do is say "ZHeatwave." I assume that's like zee-heat wave, but maybe more like zeatwave or "sheatwave" but with a z instead of s. In any case I'm sure they'll know what you mean.
h/t Washington Post