Bake tart shells according to package directions. Cool completely.
Bring blackberries and 1 cup of water to boil over medium heat. Reduce heat, and simmer for 5 minutes till blackberries are soft.
Mash blackberries with a fork and pour through wire mesh strainer into 4 cup liquid measuring cup, using a back of the spoon to squeeze out 2 cups of juice. Discard residue. Boil at reduced heat to reduce volume to 2 cups.
Combine 1 ½ cups sugar and ½ cup of flour in a saucepan; gradually add the blackberry juice, whisking constantly until smooth. Bring to boil over medium heat whisking constantly. Reduce heat and simmer for 3 minutes or until thickened. Remove from heat. Stir in butter and 2 teaspoons of vanilla.
Spoon filling into prepared tart shells. Cool completely.
Beat whipping cream at high speed with an electric mixer until foamy; gradually add remaining 1/4 cup of sugar, beating until stiff peaks form. Fold in remaining 1/8 teaspoon of vanilla. Dollop whipped cream over tarts; Garnish with fresh blackberries and mint springs.
Yield: 8 tarts with 20 min preparation time and 25 min cooking time plus cooling time.