A magical self-saucing dessert: one batter bakes into a light lemon sponge on top with a silky lemon-curd sauce hiding underneath. Serve warm or cold, dusted with icing sugar.
Preheat the oven to 325°F (160°C). Set a kettle full of water to boil.
In a stand mixer, beat the butter with the sugar until fully incorporated, then add the flour, baking powder and salt. Beat in the egg yolks, followed by the lemon zest, lemon juice and milk.
In a separate bowl, beat the egg whites until soft peaks form.
Use a spatula to gently fold the beaten whites into the lemon mixture until just incorporated (it will still look slightly curdled). Scrape the batter into a 1½-quart (1.5 L) baking dish — an 8x8-inch glass dish works.
Create a water bath by setting the dish inside a larger baking pan and pouring hot water into the larger pan until it comes about halfway up the sides of the dish. Carefully transfer to the oven and bake about 55 minutes, or until the top of the cake is set. Cool the dish on a wire rack. Serve warm or cold, dusted with icing sugar if desired.
Notes
How it works: As it bakes, the batter separates into a light sponge on top and a silky lemon-curd sauce underneath — that's the pudding-cake magic.Soft peaks only: Don't overbeat the whites; soft peaks fold in most evenly and let the layers separate properly.The water bath matters: The bain-marie keeps the custard layer gentle and prevents curdling. Pour the hot water about halfway up the dish.Lemons: Two lemons give roughly 1 tbsp zest and 1/4–1/3 cup juice. Zest before you juice.