Preheat the oven to 300 degrees F.
Lightly butter a 9-inch cake pan (preferably a spring form pan).
In a large mixing bowl, combine the ricotta, sugar, candied orange zest, liqueur, orange zest, amaro and 3 egg yolks. Using a wooden spoon, mix until homogenous. Add the egg whites 1 by 1, mixing after each addition.
Pour the ricotta mixture into the prepared pan and bake until light golden brown, about 30 minutes. Serve hot, room temperature or cold.
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