Preaheat the oven to 180C (350F). Line the base of a cake tin with baking paper and grease the sides of the tray with butter. 

Using a stand mixer on low, beat the butter and sugar till pale, about 3 minutes or so.

Meanwhile, in a separate bowl, combine the almonds, polenta and baking powder. Once the butter and sugar is mixed, pour about a third of the dry ingredients into the stand mixer, allowing to combine before adding 1 egg. Alternate between adding the dry ingredients and an egg until all of the mixture and eggs are in the mixture, allowing to combine after each addition. 

Use a spatula to push the mixture down from the sides of the bowl and mix to combine completely. 

Beat in the lemon zest and pour the mixture into your prepared tin, using the spatula to evenly distribute the mixture. Bake the cake in the oven for about 40 minutes.

The cake will be done when it has begun to shrink away from the sides, and the top is golden brown. Remove from the oven and leave in the tin to cool slightly. 

While the cake cools, add the lemon juice and icing sugar to a saucepan on a medium heat. Stir until the sugar dissolves completely and remove from heat. 

Prick the top of the cake all over with a cake skewer, and pour the warm syrup over the cake. Use a spoon to ensure the syrup evenly distributes across the cake, as it will pool in the middle. Allow to cool before removing from the tin.