Cut the banana in half. Cut two slices with skin en keep them apart for decoration. Peel the rest of the banana and cut coarsely. Cut the pomelo in halves and squeeze the juice out of the pomelo (you can use a juicer).
Mix banana, pomelo juice, buttermilk and honey with a blender. If the drink is too thick, stir in some extra buttermilk.
Pour the smoothie in 2 glasses. Decorate the glasses with the banana slices. Sprinkle a pinch of cinnamon on top and serve the smoothie cold.