Combine the 4 frozen bananas with the frozen dragon fruit, collagen and 1/3 cup milk in a high-speed blender or food processor. Blend on high, pushing the ingredients with the tamper tool if using a blender, until smooth and creamy. Add a splash more milk, if necessary, to help blend. Taste and blend in more frozen bananas to sweeten, if desired.
Divide the smoothie among 2 bowls then smooth the tops with a rubber spatula. For more of a frozen yogurt or soft-serve consistency, freeze for 30 to 60 minutes. Add your desired toppings and serve.