MIX sugar and cream of tartar in medium heavy saucepan. ADD eggs and water; BEAT until well blended. ADD chocolate; COOK over low heat, STIRRING constantly, until chocolate is melted and temperature reaches 160°F. Do not allow to boil. REMOVE from heat immediately.

COOL quickly: SET pan in larger pan of ice water. STIR vigorously until temperature of mixture falls to 80°F to 90°F. REMOVE pan from ice water. STIR in vanilla.

BEAT butter in mixer bowl on medium speed until light and fluffy, 3 to 4 minutes. ADD cooled egg mixture; BEAT on high speed 4 to 5 minutes. POUR into pie shell. REFRIGERATE, loosely covered, at least 6 hours before serving.