In a blender, combine all ingredients except chocolate, and blend until smooth. If you used a fresh banana instead of a frozen one, you may want to add ice to make it colder. Garnish with shaved or chopped dark chocolate. Enjoy!
Notes
Using full-fat coconut milk is key to making this smoothie resemble a shamrock shake. You can, however, sub any type of milk if you don't have coconut milk on hand. If your coconut milk separated in the can, blend it together by itself first to get it smooth, then measure out the cup you need for the smoothie.
Other topping ideas include shredded coconut and chocolate chips.
A little bit of peppermint extract goes a long way. I recommend starting with exactly ⅛ teaspoon (measure it out) and then adding more if desired.
If you don't have peppermint extract, sub 5 to 7 leaves of fresh mint. It works great!