Preheat the oven to 300 degrees F. In a large bowl, blend the ricotta until smooth. Then whisk in the sugar and flour. Next, whisk in the eggs one at a time. Then finish with the orange zest, liqueur, vanilla, salt and cinnamon. Once mixed, pour the batter into a sugared springform pan bake until set and golden brown, 1 hour 15 minutes.
Remove and allow to cool for 30 minutes before refrigerating. Portion into 9 to 16 slices.
You can substitute 1 pound cream cheese for half of the ricotta.