In a cheesecloth-lined strainer placed over a bowl, strain the ricotta for 12 hours or overnight in the refrigerator.
With the rack in the middle position, preheat the oven to 325°F (165°C). Butter and line an 8-inch (20 cm) springform pan with parchment paper.
In a bowl, combine the strained ricotta, ¾ cup (160 g) of the sugar, the flour, egg yolks, lemon zest and juice with an electric mixer until smooth. Set aside.
In another bowl, beat the egg whites and the remaining sugar (¼ cup/55 g) with an electric mixer until stiff peaks form.
With a spatula, gently fold the meringue into the ricotta mixture. Spoon into the prepared pan.
Bake for about 50 minutes or until a thermometer inserted into the centre of the cake reads 150°F (65°C). Let cool. The cake will have a tendency to sag or sink in the middle. Cover with plastic wrap and refrigerate for 4 hours or overnight.
When ready to serve, unmould and dust with icing sugar. Enjoy as is or top with Limoncello Lemon Confit, if desired.