1. Preheat oven to 350°F. Grease and flour two 9-inch round cake pans.
2. Prepare, bake and cool cakes following package directions for basic recipe.
3. Place one cake layer on serving plate. Spread with 1/3 cup strawberry jam. Arrange 1 cup blueberries on jam. Spread half the whipped topping to within 1/2 inch of cake edge. Place second cake layer on top. Repeat with remaining 1/3 cup strawberry jam, 1 cup blueberries and remaining whipped topping. Garnish with strawberry slices and remaining 1/2 cup blueberries. Refrigerate until ready to serve.
Tip
For best results, cut cake with serrated knife; clean knife after each slice.