Melt butter, whisk in sugar, vanilla and zest. Whisk in yolks and lemon juice. Whisk in flour, then milk. Beat whites to stiff peaks, fold into batter. Bake in 5- 6 cup dish in water bath at 180°C/350°F (160°C fan) for 35 min (ramekins 30 min).

Preheat oven to 180°C/350°F (160°C fan-forced).

Baking vessels (Note 1) - You will need a 5 to 6 cup ceramic or glass baking dish (1.25 - 1.5 litres / quarts), or 6 x 200 ml / 7oz ramekins. No need to grease it.

Barely melt butter - Put the butter cubes in a heatproof bowl and microwave in 20 second increments until almost melted. Then whisk by hand to finish melting using the residual heat - it will look milky rather than a yellow oil. (Note 2)

Make batter - Add the sugar, vanilla and lemon zest into the butter. Whisk to combine. Add lemon juice and egg yolks, whisk to combine. Add flour and whisk until lump free. Add milk and whisk. The batter will be very thin!

Beat whites - In a separate bowl, beat the egg whites until stiff using a handheld beater or stand mixer fitted with the whisk attachment. You want upright rather than floppy Santa hats!

Fold through a big scoop of the fluffed egg whites into the batter using a rubber spatula. Then add the remaining egg whites in and fold through, don't stir vigorously (Note 3 for what I mean by this). Little lumps of egg whites is fine.

Fill - Gently pour the batter into the baking dish and put it in a larger roasting pan. Ramekins: Use a ladle, and make sure to scoop from the bottom so every ramekin gets even amounts of foamy batter (which sits on the surface), this turns into the cake. Fill to 1 cm / 0.2" from the rim. Transfer to a pan for water bath.

Bake in water bath - Fill pan with boiling water until it comes halfway up the side of the baking dish. Bake 35 minutes for a single dish or 30 minutes for ramekins, until the surface is golden (it will feel jiggly if you prod it because of the sauce underneath).

Rest for 5 minutes, then dusting with icing sugar. Scoop out to serve, ensuring everyone gets their share of the lemon custard sauce! Top with a dollop of whipped cream or creme fraiche, and decorate with little lemon slices, raspberry and mint sprig if desired.