Place all ingredients (frozen banana, milk, cocoa powder, protein powder, nut butter, vanilla, sweetener, and ice) into a high-speed blender.
Secure the lid and blend on high for 45–60 seconds until completely smooth and creamy.
Stop the blender, scrape down the sides with a spatula if needed, and blend again for a few seconds.
Taste and adjust sweetness if necessary by adding more honey or maple syrup.
Pour the smoothie into a tall glass.
Garnish with a sprinkle of cocoa powder, a few chocolate shavings, or a banana slice.
Serve immediately and enjoy!