For the filling:
Step 1: Preheat the oven at 2000C. Cook the pasta shells in plenty of lightly salted boiling water until al dente. Drain well (make sure you empty the shells of water) and leave it to cool.
Step 2: To make the filling, mash the ricotta with a fork, stir in the diced mozzarella, parmesan and some salt and pepper to taste and mix well.
Step 3: Shape the mixture into 16 balls, wrap each ball in a basil leaf and place in a cooled pasta shell.
Step 4: Pour a layer of the tomato sauce over the bottom of an ovenproof dish and place the filled shells on top.
Step 5: Pour over the remaining tomato sauce with basil sauce, sprinkle over the parmesan and top with slices of mozzarella. Cover with aluminium foil and bake for 35 minutes.
Step 6: Remove the foil and bake uncovered for 5 minutes. Serve immediately