Line the base of a 23cm springform cake tin with baking parchment and grease sides lightly with butter.
Preheat oven to 180°C/gas mark 4/ 350°F.
By hand in a bowl with a wooden spoon, or using a mixer, beat butter and sugar till pale and whipped.
200 g soft unsalted butter, 200 g caster sugar
Mix together almonds, polenta and baking powder. Beat some of this into butter-sugar mixture, followed by 1 egg. Alternate dry ingredients and eggs, beating all the while.
200 g ground almonds, 100 g fine polenta, 1 ½ tsp baking powder, 3 eggs; large
Beat in lemon zest and pour/ scrape the mixture into the prepared tin. Bake for about 40 minutes.
2 lemons (zest)
It may seem wobbly but if cooked, the cake will have begun to shrink away from the tin. A skewer should come out cleanish. Leave in its tin on a wire cooling rack.
In a smallish saucepan, boil together lemon juice and icing sugar until the icing sugar dissolves.
2 lemons (juice), 125 g icing sugar
Gently prick the cake top all over with a skewer and pour the warm syrup over. Leave to cool before taking it out of its tin.