Preheat the oven to 180C/350F/Gas Mark 4. Grease and line an 11 x 7 inch/28cm x 18cm brownie/traybake tray with parchment paper.
Place the butter, sugar and lemon zest into the electric mixer and beat for 3-4 minutes until it has a pale, creamy consistency. Whisk in the eggs and milk and then sift in the flour and baking powder and beat for another 2-3 minutes.
Transfer into the prepared cake tin and bake in the oven for 40-45 minutes or until a skewer inserted in the centre comes out clean.
Allow to cool on a wire rack.
Meanwhile make up the topping.
Mix the lemon juice and most of the zest with the icing sugar and beat until a runny consistency has been achieved.
Extra lemon juice or boiling water can be used to loosen further if required.
Drizzle or spread the icing over the top of the cake and then cut the cake into 10-12 slices and garnish with some leftover lemon zest.