Place the egg whites and cream of tartar in a bowl and beat with an electric mixer on high speed for 2 minutes or until soft peaks form. (When the beaters are lifted out of the bowl, they form peaks that fold over.) Add the sugar and beat for 1 minute or until thick and glossy. Form 4 mounds of meringue in an 8 or 9 inch shallow baking dish and bake for 10 to 12 minutes or until golden brown on the edges.
FILLING: Place the sugar, cornstarch salt and water in a small saucepan and cook over medium high heat, stirring frequently, for 5 minutes or until it begins to boil. Boil for 1 minute. Place the egg yolks in a small bowl and beat slightly. Very slowly whisk some of the hot mixture into the eggs to temper them. Pour the eggs into the pan and cook over low heat, stirring constantly, for 1 minute or until the mixture barely begins to bubble. (Do not allow the pudding to boil or the eggs will curdle.) Remove the pan from the heat and stir in the butter and lemon juice.
Carefully pour the pudding around the meringue. The meringue may loosen on their own and pop to the top of the pudding. If they do not, carefully run a knife or spatula around them to loosen them. Refrigerate for at least 1 hour, or until ready to serve.