Mix together sugar and cocoa powder in a small saucepan. Add 1/2 cup milk to sugar mixture; stir constantly over medium-low heat 3 minutes or until mixture is hot and sugar is dissolved. Pour mixture into a blender. Add remaining 1 1/2 cups milk, chocolate syrup, vanilla, yogurt and ice cubes to cocoa mixture in blender. Process until smooth. Pour into glasses and sprinkle with chocolate shavings or additional cocoa powder.