Pour milk and espresso into blender jar. Add frozen banana and peanut butter. Add about a half a cup of ice. Cover with lid and press auto smoothie button.
Taste smoothie and add agave syrup as needed to sweeten. Add additional ice if thicker consistency is desired. Add chopped dark chocolate and pulse for a couple seconds until chocolate bits are distributed (and added ice is blended).
Remove blender from machine, open spout and pour into 2 glasses. Serve immediately. Garnish with a banana slice and additional chocolate chunks.