Carefully line the base of a 23cm springform cake tin with baking parchment and grease its sides lightly with a little extra soft butter. Then heat your oven to 350°F / 180°C / Gas mark 4.
In a bowl cream the butter and sugar, in a separate bowl whisk up eggs. Mix together the polenta, almonds, and bicarb, and beat one dessert spoon of this into the butter-sugar mixture. This will help stop the mixture splitting.
Add a little of the egg and beat in followed by more of the almond and polenta mix. Continue until the eggs and polenta / almond mix are fully incorporated, beating all the while.
Finally, beat in the lemon zest and zest then spoon into cake tin and bake in the oven for about forty minutes.
When cooked a small sharp knife should come out clean and the edges of the cake will have begun to shrink away from the sides of the tin. Allow to cool slightly before turning on to a wire rack.