Preparation / Directions:
Melt chocolate in bowl set over simmering water. Add sugar and mix well. Stir in butter 1 piece at a time and blend well. Remove bowl from over water and add egg yolks 1 at a time, mixing well after each addition. Beat in cocoa. Let cool 5 minutes, stirring often.
Beat egg whites with salt and cream of tartar until stiff peaks form. Gently stir 1/3 of whites into chocolate mixture, then fold remaining whites into chocolate mixture; DO NOT BEAT. Whip heavy cream to soft peaks still pourable. Carefully fold cream into chocolate mixture until well blended. pour into 9x5-inch glass loaf pan or 1 1/2-quart mold; DO NOT USE METAL. Chill at least 10 hour or overnight.
Just before serving, moisten a towel with hot water. Wrap towel around pan or mold and invert onto platter. Slice marquise into serving portions.