Heat oven to 325°F. Grease an 8-inch square baking dish and line the bottom with parchment.
Boil water in a kettle for a bain-marie. Set a larger roasting pan on the middle rack.
Melt the butter and let it cool until just warm to the touch.
Whisk egg yolks with sugar in a large bowl until pale and slightly thick.
Whisk in the melted butter, lemon zest, vanilla, and salt until smooth.
Sprinkle in the flour and whisk just until no dry spots remain.
Whisk in the milk, then the lemon juice, until the batter is thin and silky.
Beat egg whites to soft peaks in a clean bowl. Fold into batter in three additions.
Pour batter into the prepared dish. Set it inside the roasting pan and add hot water halfway up the sides.
Bake 35 minutes, until golden on top and slightly jiggly in the center. Cool 15 minutes, dust, and serve.