In this vibrant yet cozy dessert, the batter separates into two layers: one like a thick pudding, one like an airy cake. Both are divine, and perfectly complementary.
Preheat oven to 350°. Butter an 8x8-inch baking dish; then set dish into a larger pan (this will be the water bath for the pudding). Meanwhile, bring about 2 quarts of water to a simmer.
In a standing mixer fitted with a whisk attachment, whip egg whites until medium peaks form (they should just hold their shape when you lift the whisk). Spoon whites into another bowl and set aside.
Wipe out the mixing bowl and fit mixer with a paddle attachment. Cream sugar and butter until sugar looks wet. Add egg yolks, flour, and salt. Mix until smooth.
Add milk, cream, lemon juice, and lemon zest and stir. Working by hand, gently stir a third of the egg whites into the mix until mostly blended; then fold in remaining whites until combined (there may be some streaks, and that’s fine). Pour mixture into prepared baking dish; then pour simmering water into the larger pan so that it comes up to the level of the pudding. Bake until top is golden brown and puffed, about 30–40 minutes. Garnish with lemon zest if you like.