Heat the oven to 170C/335F/gas mark 3-4.
Line the bottom and sides of an 8 1/2 inch or 22cm cake tin twith nonstick baking parchment.
In a large bowl, using an electric mixer, beat the melted butter, condensed milk, 125g of the caster sugar, egg yolks, 100ml of the lemon juice (saving the other 25ml in a cup) and two-thirds of the zest until smooth and the sugar dissolved.
In a separate bowl, beat the egg white with the remaining 25g of sugar to a soft meringue.
Sift together the flour and baking powder, beat this into the lemon mixture until smooth, then fold through the meringue quickly and evenly.
Tip the mixture into the cake tin and smooth the top.
Bake for 40-45 minutes until an inserted skewer comes out clean.
When cool, beat the remaining lemon zest with the icing sugar and the reserved 25ml lemon juice (adding a bit of water to soften it if necessary), and slather this over the top of the cooled cake.