On a cutting board, slice banana and enough cucumber to fill one cup.
Pour water into blender (it is important to add water first so that everything will blend smoothly). Add cucumber, banana, and mint**, and blend thoroughly. Then add cocoa powder and stevia and blend on low until mixed through.
Pour into a glass and serve. This makes a good snack or light meal. For summertime, add ice to make it more like a smoothie and less like a shake.
**En lieu of fresh mint, you can substitute 1/2 cup of mint tea for 1/2 cup of water