Preheat the oven to 150˚C. Line and grease a 25cm square tray and line with parchment paper.
Place the butter and sugar into a mixing bowl and mix until light and fluffy.
Stir in the lemon zest at this stage also. Next, add in the eggs and sieve in the flour and baking powder and stir until fully incorporated. Pour the mixture in the tray.
Place in a preheated oven and bake for 50-60 minutes or until a skewer inserted in the centre comes out clean. Remove from the oven and allow to rest for 5 minutes.
In the meantime, in a saucepan, combine the water, lemon juice, lemon zest and caster sugar . bring to simmer for 5 minutes until clear and reduced by half. Remove from the heat and set aside to cool.
Once cooled, with a skewer, create holes on the cake, dab with the syrup and wait for 5 minutes to settle.
To decorate, in a bowl, combine the lemon zest and juice with the icing sugar until smooth. Drizzle over the top of the cake.
If the drizzle is not loose enough in consistency add a tiny amount of hot water and beat that into the mixture to achieve a running consistency.
Set aside to cool and dry for 1 hour if possible. Slice the cake into square and serve within a few days.