Line the cake pan with baking paper. If your pan comes in two pieces like mine, cover the bottom completely with aluminum foil. Preheat the oven to 150 or 300℉. Bring some water to a boil for a water bath. Sift the flour into a big bowl.
Put the unsalted butter and milk in a small saucepan, heat to around 80℃/175℉.
Remove from the heat and add to the sifted flour, mix to combine well. It will look like it has separated, but that’s totally fine, it will combine well when you add the egg yolks.
Add the egg yolks and mix well to combine. Then mix in the vanilla extract.
Add the sugar to the whites, and start whipping at medium speed. Whip until medium soft peaks form.
Add half of the meringue to the egg yolk mixture, mix with a rubber spatula. Before adding the rest of the meringue, whisk it a little by hand to make it smooth again, because it has separated a bit. Meringue will start to separate as soon as you finish whisking. Then add to the mixture, folding gently from bottom to top. Be careful not to over mix.
Pour into the prepared pan, and tap once to remove big air bubbles in the cake batter. Place the paper towel or kitchen towel onto a baking tray to prevent a deep tray from slipping, then place the deep tray for a water bath. Place the cake onto it, and pour the hot water into the deep tray.
Bake in the preheated oven and bake for 50min.
After taking it out from the oven, remove from the pan and remove the paper. This prevents the cake from getting wrinkles on the sides of the cake.