Sprinkle gelatin over 1 tablespoon cold water; let stand 1 minute. Combine egg yolks, 3 tablespoons sugar, lemon rind, and lemon juice in a small saucepan. Stir in milk. Cook over medium-low heat, stirring constantly, 20 minutes or until thickened. Add gelatin mixture; stir until gelatin dissolves.
Transfer mixture to a small bowl. Cover and chill 1 hour or until mixture mounds slightly when dropped from a spoon.
Beat egg whites and cream of tartar at high speed of an electric mixer until stiff peaks form.
Combine remaining 1/4 cup sugar and 2 tablespoons water in a small saucepan. Bring mixture to a boil; cook, without stirring, until candy thermometer registers 238° (about 7 minutes).
Gradually pour sugar mixture in a thin stream over egg white mixture while beating constantly at high speed. Continue to beat until mixture is cool and set. Fold egg white mixture into lemon mixture. Spoon into a 2 1/2-cup souffle dish. Cover and chill at least 4 hours.