In a blender, add about half the soy milk and the spinach. Puree until the spinach dissolves, about 30 seconds. Add the remaining soy milk, banana chunks, vanilla extract, flax seed, and chocolate syrup, if using, and blend until the mixture reaches a smooth consistency, another 30 seconds. Divide mixture between two chilled glasses with straws.
Cook's Note: If the smoothie is looking a little too green from the spinach, add a squeeze of chocolate syrup to enhance the smoothie's brown color.