1. Whisk together sugar and flour in a 3-qt. heavy saucepan; add buttermilk and eggs, and whisk until blended. Cook over medium heat, whisking constantly, 7 to 8 minutes or until a pudding-like thickness. Remove from heat, and stir in vanilla bean paste. Cover and chill 4 to 24 hours.
2. Meanwhile, bake frozen tart shells according to package directions, and cool completely (about 30 minutes). Spoon custard into tart shells, and top with desired toppings just before serving.