Take a large pot, pour in brown sugar, corn flour, cocoa powder and salt, and add soybean milk while stirring until it is completely mixed. Cook over medium heat, stirring while cooking, and then cook for 1 min until the mixture thickens. Remove the pan from the fire, add mint and vanilla extract while it is hot, and stir. Cool at room temperature and mix in lotus root slices. Spoon the pudding into 4 molds, cover and refrigerate for 2 hours, or until it is solidified.