In a large bowl and using an electric hand whisk or electric mixer, beat the butter and sugar until pale and fluffy, then add the ground almonds and combine.
Beat in the eggs one at a time.
Next, add the lemon juice and zest, polenta, baking powder and a pinch of salt.
Spoon the mixture into a 9in round cake tin (not loose bottomed, 2in deep, lightly buttered and lined with baking parchment) and level out with the back of a spoon.
Bake on the centre shelf of the oven for 1 hour 10 mins or until the cake is firm to the touch and a metal skewer comes out clean.If the cake appears to be browning too much towards the end of the cooking time, cover the top loosely with aluminium foil.
When the cake is nearly ready, make a syrup by gently heating the lemon juice, zest and the sugar until the sugar has dissolved, then bubble the mixture for a couple of minutes until slightly syrupy.
When the cake is cooked, take it out of the oven and drizzle two-thirds of the warm syrup over the top of the cake while it is still in the tin.
Leave the cake to cool before turning out and slicing.