Place all ingredients in a blender and mix, stopping once to scrape the cocoa from the sides of the blender container. Continue mixing until smooth.
If your banana isn't ripe, you may need to add sweetener.
I've also made this smoothie with cashew milk, which works, but I prefer the flavor of almond milk here. I've never tried it with oat milk. You could even use dairy milk, if you don't need this to be vegan.
For a fruity variation, you can add ¼ cup of blueberries. Any more than that will overpower the chocolate.
You can use peanuts instead of pecans for a delicious, chocolate-peanut butter variation.