Preheat oven to 160C. Using an electric mixer, beat butter and sugar until pale and creamy, then add eggs one at a time, beating well after each addition. Fold in almond meal, then add polenta, lemon rind and juice. Add ricotta and honey and stir until just combined, trying to keep walnut size lumps of ricotta throughout mixture. Spoon mixture into a lightly greased and baking paper-lined 26cm-diameter cake pan. Bake for 1 hour or until golden and a skewer withdraws clean when inserted. Cool completely in pan.
For roasted plums, place plums cut-side up in a roasting tray. In a saucepan, combine 300ml water and remaining ingredients, bring to the boil and pour over plums. Roast for 20-25 minutes or until tender. Remove plums, return liquid to saucepan and slowly bring to the boil. Cook for 30 minutes or until reduced to 1 cup.
Dust cake with icing sugar. Serve wedges of cake with plums and syrup to the side.
