In a blender or food processor combine strawberries and 1/4 cup sugar. Cover and blend or process until smooth; set aside.
In a large saucepan, combine the water and masa harina. Cook and stir over medium heat for 3 to 4 minutes or until thickened.
Stir in buttermilk, 1/2 cup sugar, vanilla bean (if using), cannella stick, and pureed strawberries. Cook over low heat for 5 minutes, stirring frequently. Stir in vanilla (if using). (The soup should be thick, but pourable. If necessary, thin with a little milk.) Cover and chill for at least 4 hours.
Before serving, remove vanilla bean and cannella stick. Using the tip of a small sharp knife, scrape seeds from vanilla bean; return seeds to soup. Ladle soup into small dessert bowls. Garnish with mint sprigs.