- In a sieve lined with cheesecloth, over a bowl, drain the ricotta for 12 hours or overnight.
- With the rack in the middle position, preheat the oven to 325 °F (170 °C). Butter and line with parchment paper a 20 cm (8-inch) springform pan.
- In a bowl, combine the drained ricotta, 3/4 cup (180 ml) of the sugar with the flour, egg yolks, zest and liqueur with an electric mixer until smooth. Set aside.
- In another bowl, beat the egg whites and the remaining sugar (1/4 cup / 60 ml) with an electric mixer until stiff peaks form.
- With a spatula, gently fold the whites into the ricotta mixture. Pour into the prepared pan.
- Bake for 50 minutes or until a thermometer inserted in the centre of the cake reads 150 °F (65 °C). Let cool for 1 hour. Refrigerate for 4 hours or overnight. Dust with icing sugar before serving.