Preheat your oven to 180°C / 356°F and line a baking sheet with baking paper.
In a large bowl cream together the butter and the sugar, until light and fluffy.
Add the eggs and keep beating for 1-2 minutes.
Add the flour, the vanilla and the baking powder. Add the zest of all three lemons and the juice of one lemon.
Give it a final beat and spread it evenly onto the baking sheet.
Bake for 20-25 minutes or until a wooden toothpick comes out clean.
Meanwhile prepare the drizzle. Mix the juice of one lemon with 1 cup of water.
When the cake comes out, let it cool for 5 minutes.
Poke it with a wooden skewer all over and then carefully spoon on the drizzle.
When the cake has cooled off, make the glaze by mixing the confectioners sugar with the juice of the remaining lemon. Carefully add some water, just a little bit at a time until you have a nice thick glaze. Spread it evenly on the cake and decorate to your liking.
Cut into bars while the glaze is still wet.