Preheat the oven to 180C/350F/Gas 4.
Grease 2x23cm/9in loose-bottomed cake tins with oil and line with a 23cm/9in diameter disk of baking parchment or greaseproof paper.
In a medium sized glass bowl gently melt the chocolate with the butter over a pan of gently simmering water or on the defrost mode of the microwave.
In a large bowl whisk the yolks with three quarters of the sugar until light and ribbony.
In a separate glass bowl whisk the whites until stiff, add the remaining sugar and whisk for 1 minute.
Fold the slightly cooled melted chocolate, polenta and walnuts into the egg yolks. Add a quarter of the whisked egg whites and stir briefly then fold in remainder with a metal spoon.
Pour the mixture into the prepared tins and bake for 25 minutes, until firm to the touch and just cooked.
Remove the cakes from the oven and allow to cool in the tins. Remove and place on a wire rack.
Sandwich the cakes together with three quaters of both the crème fraîche and the cherries.
Decorate the top of the cake with the remaining crème fraîche and cherries and sprinkle with the prepared chocolate shavings.
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