Whisk the butter and sugar together until really light and fluffy, at least 5 minutes. Crack the eggs into a bowl and whisk lightly with a fork. Add a literally a tablespoon of egg at a time to the butter mixture, scraping down the sides of the bowl occasionally. It's worth taking time with this as this will ensure a super light cake.
Combine the flour with the ground almond and zest of the lemons. Fold in 3 additions to the cake batter. Pour into a greased and lined 20cm cake tin or a 900g loaf tin. If you're adding berries just sprinkle a handful on the top, pushing them in slightly.
Bake at 160C fan for 40-42 minutes in the cake tin or 50 minutes in a loaf tin. Test to see if it’s ready by inserting a cocktail stick in the centre of the cake, it should come out clean.
Juice the lemon and add the sugar, stir and pour over the baked cake as it comes out of the oven.
Cool in the tin for 20 minutes before placing on a plate.