Sift the flour and baking powder into a mixing bowl. Stir in the polenta and set aside.
Whisk the eggs, egg whites and sugar together in another bowl, using an electric whisk, for 3–4 minutes until pale and very thick. Fold in the polenta mixture, lemon rind and juice, vegetable oil and buttermilk to form a smooth batter.
Pour the mixture into a greased and base-lined 25 cm (10 inch) spring-form cake tin. Bake in a preheated oven, 180°C (350°F), Gas Mark 4, for 30 minutes until risen and firm to the touch. Leave to cool in the tin for 10 minutes then loosen the edges, turn out on to a wire rack and peel off the lining paper. Leave to cool.
Meanwhile, prepare the red wine strawberries. Put the wine, vanilla pod and sugar in a saucepan and heat gently to dissolve the sugar. Increase the heat and simmer for 10–15 minutes until reduced and syrupy. Leave to cool then stir in the balsamic vinegar and strawberries.
Cut the cake into slices and serve as a dessert with the strawberries and their syrup.