1) Preheat the oven to 180°C/gas mark 4. Grease and flour two (22x11x7 cm) loaf pans.
2) Cream the butter and 400g of the caster sugar in the bowl of an electric mixer fitted with the paddle attachment. The key to great cakes is beating the butter and caster sugar until the mixture is light and fluffy, about 5 minutes.
3) With the mixer on medium speed, add the eggs one at a time and the lemon zest (break the eggs into a dish first to avoid adding a bad egg or shells to the batter).
4) Sift together the plain flour, baking powder, bicarbonate of soda and salt in a bowl. Separately, combine 60ml of the lemon juice, the buttermilk and vanilla. Add the plain flour and buttermilk mixtures alternately to the creamed butter, beginning and ending with the plain flour.
5) Divide the mixture evenly between the pans, smooth the tops and bake for 45 minutes to 1 hour, until a wooden skewer comes out clean.
6) Combine 100g of the caster sugar and the remaining lemon juice in a small saucepan and stir over low heat until the caster sugar dissolves.
7) When the cakes are done, cool for 10 minutes, invert them onto a rack set over a tray and spoon the lemon syrup over the cakes. Allow to cool completely.
8) Pour the glaze over the tops of the cakes and allow to drizzle down the sides.
Combine the icing sugar and lemon juice in a bowl, mixing with a wire whisk until smooth.
Cook's note: To freeze these cakes, prepare everything except the glaze. Wrap them well in cling film and freeze. When you're ready to use them, defrost and glaze.