In medium saucepan sprinkle gelatin over 1 c. milk; allow to stand 2 minutes to soften. Stir together sugar and cocoa; add to mixture in saucepan. Cook over low heat, stirring constantly until mixture boils.

Remove from heat; add butter, stirring until melted. Blend in remaining 2/3 c. milk and vanilla. Cool; chill, stirring occassionally, until mixture begins to set. Beat whippping cream until stiff; carefully fold into chocolate mixture. Garnish as desired.