Grease a 1.2 litre (2 pint) pie dish lightly, then stand the dish in a roasting tin. Put the rest of the butter in a mixing bowl with the sugar and lemon rind. Whisk the egg whites in a separate bowl until they are softly peaking. Using the still dirty whisk, beat the butter, sugar and lemon rind until light and fluffy, then mix in the flour and egg yolks.
Mix in the milk and lemon juice gradually until only just mixed. The mixture may appear to separate slightly but this will disappear during baking.
Fold in the egg whites, then gently pour the mix into the greased dish. Pour hot water from the tap into the roasting tin to come halfway up the sides of the dish.
Cook in a preheated oven, 190°C (375°F), Gas Mark 5, for about 25 minutes until slightly risen, golden brown and the top has begun to crack. Insert a knife into the centre – the top two-thirds should be soufflé-like and the bottom third a saucy, custard-like layer. If it's very soft in the centre, cook for an extra 5 minutes.
Dust the top with a little sifted icing sugar, if liked, then serve immediately spooned into shallow bowls. Don't leave the dessert to stand or the topping will absorb the sauce.