Preheat oven to 325 degrees F.
Generously grease and flour a springform pan.
For graham cracker crust, crush graham crackers in food processor until consistency of bread crumbs; stir with melted butter until mixture holds shape when squeezed. If needed, add a little more melted butter.
Firmly press graham cracker crust into bottom and along bottom sides of spring form pan; set aside.
Sift together flour and sugar in small mixing bowl.
In medium bowl, stir together ricotta cheese and orange zest. Slowly fold in flour mixture with spatula until well blended and smooth.
Add eggs, one at a time, continuing to blend with spatula. Stir in vanilla extract.
Pour batter into spring form pan and gently smooth top with spatula.
Bake for 80-90 minutes, until cake is firm all the way to top center. Cool completely and refrigerate for 2-3 hours to set.
Once set, run a warm knife around the edge of the springform pan and gently release.
Top with powdered sugar, chocolate chips, fruit or the topping of your choice.