Beat egg yolks and vanilla about 5 minutes or until thick and lemon colored. In saucepan combine 1/2 cup sugar and gelatin. Stir in coffee and 3/4 cup cold water. Cook and stir until gelatin dissolves. Stir in chocolate. Cook and stir until mixture boils and is smooth. Pour gelatin mixture in thin stream into yolk mixture, beating constantly. Chill to consistency of corn syrup, stirring occasionally.
Remove from refrigerator (gelatin mixture will continue to set). Immediately beat egg whites until soft peaks form. Gradually add 1/3 cup sugar, beating until stiff peaks form. When gelatin is partially set (consistency of unbeaten egg whites), fold in stiff-beaten whites. Chill until mixture mounds when spooned. Spoon into pastry shell. Cover and chill 8 hours or until set.