Peel banana, cutting off and discarding any bruised spots. Slice into workbowl of food processor fitted with steel blade OR blender container. Cover; blend/puree at high speed until smooth, scraping container with spatula if necessary to be sure all of banana is pureed. Add yogurt and optional chocolate syrup. Cover and blend/puree at high speed briefly, until all ingredients are combined. Serve immediately, or cover and refrigerate for as long as overnight.