1.Preheat oven to moderate. Grease deep 22cm-round cake pan; line base and side with baking paper.
2.Combine dates and liqueur in small bowl; stand 15 minutes.
3.Meanwhile, roast nuts on oven tray in moderate oven about 10 minutes. wrap warm nuts in tea-towel, rub to remove skins. Chop nuts coarsely.
4.Reduce oven temperature to moderately slow.
5.Combine flour, baking powder, polenta, sugar, ricotta, butter and the water in large bowl; beat on low speed with electric mixer until just combined. Increase speed to medium; beat until mixture changes to a lighter colour. Using wooden spoon, stir in nuts and undrained date mixture.
6.Make ricotta filling. Combine ingredients in medium bowl, stir with wooden spoon until smooth.
7.Spread half the cake mixture into prepared pan; using metal spatula, spread ricotta filling over cake mixture in pan then cover with remaining cake mixture.
8.Bake cake in moderately slow oven 45 minutes; cover tightly with foil, bake about 1 hour. Discard foil, stand cake in pan 10 minutes before turning onto wire rack; turn top-side up to cool.
Chopped seeded prunes or raisins can be substituted for dates. Brandy or rum can be substituted for Grand Marnier. Hazelnuts also can be replaced with any other variety of nut you like. Just be certain you select a citrus fruit that complements the flavour of the liqueur and nuts you choose. For example, Cointreau, lemon rind and walnuts marry well, as do brandy, mandarin and pecans.