This lemon sponge pudding separates as it cooks into a layer of cake and a delicious custardy layer of curd; the pudding is served with rhubarb sauce.
Heat oven to 325 degrees. Butter six 5-ounce ramekins or custard cups; coat them with 2 tablespoons sugar.
In the bowl of an electric mixer fitted with the paddle attachment, beat 2 tablespoons butter, 1/4 cup sugar, salt, and zest on medium speed until well combined, 1 to 2 minutes. Add yolks; beat until creamy, 2 to 3 minutes. Add milk, lemon juice, and flour; stir to combine.
In a separate bowl, beat egg whites to soft peaks; gradually add 1/2 cup sugar. Beat until stiff and glossy. Gently whisk in egg-yolk mixture until combined. Ladle into ramekins; place in 9-by-13-inch baking pan; fill pan halfway with hot water. Bake until puddings have set and are golden on top, 35 to 40 minutes. Transfer to a wire rack to cool. Chill puddings until ready to serve, up to 2 days. To unmold puddings, dip ramekins in warm water, and run a paring knife around the edges to help loosen; unmold onto plates.
In a medium skillet, combine rhubarb, remaining 1/4 cup sugar, and 2 tablespoons water. Cook over medium heat until rhubarb is tender, 5 to 7 minutes. Transfer from heat; cool to room temperature. Serve with lemon sponge pudding.
