Garnish with mint sprig. Combine first 3 ingredients in a saucepan and bring to a boil over medium heat. Cook for 45 seconds, or until sugar is dissolved. Remove from heat and allow to cool. In a food processor, combine berries and puree until smooth.
Strain berries to remove seeds.
Cover and freeze for 1 hour. In an extra large mixing bowl, allow egg whites to come to room temperature for 30 minutes. Meanwhile sift together powdered sugar and flour 3 times. Add cream of tartar and vanilla to egg whites. Beat with electric mixer until soft peaks form.