Sprinkle gelatin over 2 cups evaporated milk in a medium saucepan; let stand 5 minutes. Cook over medium heat until gelatin dissolves and mixture just comes to a boil. Gradually stir about 1 cup of hot milk mixture into egg substitute; add to remaining hot milk mixture, stirring constantly. Cook, stirring constantly, 2 additional minutes (do not boil). Remove from heat. Stir in remaining 2 cups milk and vanilla.
Combine cocoa and sweetener in a large bowl. Gradually add hot milk mixture, stirring until smooth. Stir in chopped candy. Chill approximately 30 minutes just until cold, stirring occasionally (do not overchill).
Pour chocolate mixture into freezer container of a 4-quart hand-turned or electric freezer. Freeze according to manufacturer's instructions. Pack freezer with additional ice and rock salt; let stand at least 1 hour before serving.