This lemon-lime pudding is combined with a rhubarb compote.
Preheat oven to 325 degrees. Butter four 6-ounce glass custard cups; coat with sugar. Line a baking pan with a cloth towel, and set aside.
Make puddings: In the bowl of an electric mixer fitted with the paddle attachment, combine 2 tablespoons butter, 1/2 cup plus 1 tablespoon sugar, and salt; mix on medium speed until crumbly. Mix in lemon and lime zests and egg yolks. Mix in flour, then lemon and lime juices and milk.
In a medium bowl, beat egg whites until stiff. Gently whisk in egg-yolk mixture. Ladle into custard cups. Place cups in prepared baking pan, and fill pan halfway with hot water. Bake until puddings have set (tops may crack), about 25 minutes. Cool puddings on a wire rack, 30 minutes. Chill or let stand at room temperature.
Meanwhile, make rhubarb sauce: In a small skillet, combine rhubarb, remaining 3 tablespoons sugar, and 1 tablespoon water; cook over medium-high heat until tender, about 4 minutes. Let cool. Invert puddings onto plates, and serve chilled or at room temperature with rhubarb sauce.