Preheat the oven to 170C/150C Fan/Gas 3½. Grease a loaf tin with a little butter and line with baking paper.
Tip the remaining butter and caster sugar into a food processor and blend for a few seconds. Add the eggs one at a time, blending after each addition. Add the flour and lemon zest and blend until smooth.
Pour the batter into the lined baking tin, flatten the top with the back of a spoon and bake for 40 minutes.
Meanwhile, stir together the lemon juice and icing sugar. When the cake has cooled a little, poke holes in the top using a skewer, chopstick or a piece of raw spaghetti. Drizzle over the icing, set aside for a few minutes, and serve.