Preheat the oven to 350.
Put one 8-ounce package of cream cheese, 1/3 cup of the sugar, and all the cornstarch in a large bowl and beat with an electric mixer on low until creamy. This takes about 3 minutes.
With the mixer still on low speed, beat in the rest of the cream cheese and the ricotta cheese. Occasionally scrape the sides and bottom of the bowl.
Increase the mixer speed to high and beat in the remaining 1 1/3 cups of sugar. Beat in the vanilla. Add the eggs, one at a time. After each egg beat well and scrape the sides of the bowl.
Reduce the mixer speed and blend in the heavy whipping cream. Stop mixing as soon as the heavy whipping cream is combined.
Gently spoon the filling into the prepared 9-inch springform pan.
Place the springform pan in a large shallow pan containing hot water. The hot water should come up the sides of the springform pan about an inch.
Bake one hour at 350. The cheesecake is done when the center just barely jiggles when you shake the pan.
Cool on a wire rack for an hour.
Cover with plastic wrap and refrigerate until it's completely cold, about 4 hours. Overnight is best.
Remove the sides of the springform pan and slide the cake onto a serving plate or cake pedestal.
Cut the cake with dental floss instead of a knife. Once you make a "cut" with dental floss, you may still need a knife to cut through the crust.