This cheesecake has no crust just the butter and sugar. Preheat oven to 375. Generously butter and sugar a 9 inch springform pan.
In a large bowl whisk ricotta, egg yolks, flour, half the sugar (6 tablespoons) zest and salt until combined; set aside.
Beat the egg whites until foamy. With mixer on high,gradually add remaining sugar until egg whites are stiff and glossy. With rubber spatula, add 1/3 of egg white mixture to ricotta mixture until combined; gently fold the rest of the egg white mixture in until combined. Pour into the prepared pan and bake for 1 hour or until center is firm and top is golden brown. Test, center must be firm. Remove from oven and let cool. Serve warm or after cooled refrigerate wrapped loosely with plastic wrap. Should be at room temperature to serve. This lasts about 3 days folks.