(I'm a little kid inside, so I gave it a kid title and would probably be a great snack or lunch for kids ^__^)
Spread peanut of almond-nut butter on bread (however thin or thick you prefer, it just should be able to hold the bananas onto the bread*), place sliced bananas on in any order you like, and dist with cocoa powder. If you like, only put banana slices on one side and make a sandwich.
*Smooth peanut or almond-nut butter works the best.
*It can be pretty yummy to toast the bread before OR after you put the ingredients on.