Mix sugar, cocoa powder, corn starch and salt in a large microwave-safe bowl. Whisk in milk and egg yolks until well blended. Microwave on HIGH (100%) power for about 5 to 7 minutes, stirring every 1 to 2 minutes. Cook until pudding is thick and has boiled at least 1 minute.
Remove from microwave. Stir in butter and vanilla. Cover surface with plastic wrap. Chill 2 hours.
Layer pudding, marshmallow cream and graham cracker crumbs in individual clear bowls or stemware.