Prepare Pans: Use 6, 8 and 10 in. springform pans (3 in. deep). Place a cake circle (6, 8, 10 in.) over bottom; butter sides of pans. Wrap outside of pans with aluminum foil.
Prepare Crust: Mix 3 1/2 cups graham cracker crumbs with 3/4 cup sugar and 1 stick (1/2 cup) melted butter. Press firmly onto bottoms of prepared pans using 3/4 cup for 6 in., 1 cup for 8 in. and 1 3/4 cups for 10 in. pan.
Prepare Batter: Place cream cheese and 2/3 cup sugar in mixing bowl. Using electric mixer, cream mixture until smooth. Add eggs, one at a time, while mixing. Melt Candy Melts following package directions; cool 7-10 minutes. (It is important that candy be cooled, but still liquid; if candy is too hot, it will get lumpy when added to batter.) Add cooled candy, vanilla and whipping cream to cream cheese mixture; blend well. Pour batter into prepared pans.
Bake Cheesecakes: Place each foil-wrapped pan into a larger pan or oven-proof dish. Pour hot water into outer pans to fill 1/2 in. deep. Bake in center of preheated 300° oven: for 6 in. cake, 1 hr. 45 min.; for 8 in. cake, 2 hrs. 10 min.; for 10 in. cake, 2 hrs. 30 min. To test for doneness, gently shake pan. Top of cake should move as one solid piece with no movement in center (cakes will set completely when refrigerated). Remove from oven to cool then refrigerate overnight.