Preheat your oven to 200ºC (400ºF) or gas mark 6. Grease a 1.5-liter ovenproof dish with butter.

In a mixing bowl, combine the softened butter and sugar and beat until pale and fluffy, about 3–5 minutes.

Add the grated zest of one lemon into the creamed mixture and mix well.

Separate the eggs, adding the yolks one by one, mixing thoroughly after each addition.

Sift the self-raising flour into the batter gradually, stirring gently until smooth and lump-free.

Stir in the milk and the juice of about 3 tablespoons of lemon until well combined.

Whisk the egg whites in a clean bowl until stiff peaks form, about 3-4 minutes.

Gently fold the whipped egg whites into the lemon batter using a spatula.

Pour the batter into the prepared dish and place it inside a larger roasting tin filled with boiling water.

Bake for approximately 45 minutes, until the top is set and golden with a slight wobble.

Remove from the oven and let it sit for a few minutes before serving warm.