Preheat the oven to 180'C.
Butter a cake tin and line with baking parchment.
Split the vanilla pod in half and scrape out the seeds. Zest and juice the lemon.
Beat the sugar and the butter together until pale and fluffy and then add the ground almonds, polenta, baking powder and the vanilla seeds. One by one, beat in the eggs and then add the lemon juice and the zest. Combine well and then pour the mixture into the cake tin.
Transfer to the oven and bake for 30 minutes. Check the centre of the cake using a skewer - if it comes out clean, the cake is ready. If not, return to the oven for another 5 to 10 minutes. Leave to cool in the tin.
Zest and juice the lemon, placing both in a large bowl. Sift the icing sugar and then beat it into the lemon until thick and smooth. Once the cake has cooled, pour the icing over it and spread with a palette knife to cover the whole top.
«If you prefer, spread lemon curd over the cake as an alternative topping.»