Unfold piecrusts according to package directions. Cut each piecrust circle into six equal wedges. Working with one wedge at a time, double fold all three sides in ¼ inch. Twist the points as necessary. Flute edge of pastry to form an edge. Place on baking sheets. Prick bottoms of tart shells with a fork. Bake in a 425° oven for 8 minutes or till golden brown. Immediately divide ½ cup of the mozzarella cheese among shells.
Meanwhile, cut each tomato into six wedges. Combine remaining mozzarella cheese, Parmesan cheese, sour cream, mayonnaise, basil, and garlic powder in a small bowl. Arrange two tomato wedges on each tart. Spoon some of the cheese mixture over tomatoes in each shell. Sprinkle with freshly ground pepper.
Bake in 425° oven for 10 minutes or till cheese is melted and begins to brown. Garnish with fresh basil sprigs, if desired. Makes 12 servings.