Place egg whites in a large bowl; let stand at room temperature for 30 minutes. Add cream of tartar, salt and vanilla; beat until soft peaks form. Add sugar gradually, about 1 tablespoon at a time, and beat until very stiff. Place in a greased 9-in. round baking dish. Bake at 275° for 55 minutes or until meringue is a light cream color and is dry and firm to the touch. Cool thoroughly.
For filling, beat egg yolks in the top of a double boiler. Add sugar, lemon juice, zest and salt. Cook until thick, about 5-8 minutes. Cool. Fold in whipped cream. Spoon into shell. Refrigerate overnight. If desired, garnish with whipped cream and lemon slices.