Pre-heat oven to 350 degrees.
Slice guavas in half and remove the tough seeds inside. Mash the guava pieces together with the overripe banana. You're going for the smoothest, most gooey, liquidy texture possible as this is the only wet ingredient in this banana bread. I put everything in a bowl and mashed it together with a blunt object. This is called the "idiot's method". It works, but I'm sure you can find a smarter way.
Cream vegan butter and sugar. (This is easiest with a stand mixer but you can do it by hand if you've got the willpower.)
Add the mashed guava and banana to the creamed butter and sugar.
Sift together the flour, baking soda, salt, and cardamom.
Stir into the batter, but don't overmix. You just want to make sure it's well combined.
Stir in the vanilla.
Grease a loaf pan and dust it with a little flour.
Pour the batter into the pan.
OPTIONAL: Sprinkle the zest of half a lime over the batter. I did this to add a touch of sharpness to the flavor of the bread, and because it looked fun. It really doesn't add much, so you can omit this step. Alternately, you can slice a banana in half the long way and carefully drape it onto the top of your batter. That always looks nice, but it requires a little extra cook time. Keep that in mind.
Bake at 350 degrees for approximately 1 hour and 15 minutes. You'll want to keep an eye on your bread at this point. You'll know it's ready when it's nicely browned on top, and when you can place a toothpick through the middle and have it come out clean.
Remove from the oven, let cool for a bit, slice, and enjoy.