In medium size saucepan, mix gelatin and cocoa. In separate bowl, beat egg yolks with 1 cup of milk. Blend into gelatin mixture. Let stand 1 minute to soften gelatin. Stir over low heat until gelatin is completely dissolved, about 5 minutes. Add remaining milk, sugar substitute and vanilla. Pour into large bowl and chill, stirring occasionally, until mixture mounds slightly, when dropped from spoon. In separate large bowl, beat egg whites until soft peaks form, gradually add gelatin mixture and beat until doubled in volume,  about 5minutes. Chill until mixture is slightly thickened. Turn into dessert dishes or 1 quart bowl and chill until set. Makes about 8 servings of the 1/2 cup of each.