In a medium saucepan over medium heat, stir together the whole milk, sweetened condensed milk, and vanilla bean. Bring the mixture almost to a boil, stirring constantly. Remove from the heat and let it steep for 20 minutes. Strain and return to the pan. (Rinse and dry the vanilla bean and save it for another use.)
Put the pan on low heat, add the chocolate, and whisk until the chocolate melts. Combine the cornstarch and water to form a slurry. Add the slurry a little at a time, whisking constantly, until the soup is thick and smooth. You will know it is ready when the bubbles are gone and the chocolate has thickened, about 5 to 7 minutes.
Pour the soup into bowls and garnish with biscotti, raspberries, and mint leaves.