Wow, I know that it sounds like it is going to be super heavy. And it is heavier than a lot of milkshakes that I have drank, but it is worth every calorie.
The texture that the buttermilk adds is simply incredible. I really don't know how else to describe it, but it tastes like it sounds. The banana and the mango go really well together and they just keep adding to the flavors, while the buttermilk blends it all together and adds the desired thickness.
I would give you the recipe, but it doesn't have anything magic in it, just the buttermilk, half a ripe banana and then (I opted for canned) mango.
The just blend it up, and the more you blend the creamier it gets.