Prepare pastry shells according to package directions; set aside.
Bring chestnuts, 1 cup sugar, and 1 cup milk to a boil in a small saucepan, stirring until sugar dissolves; reduce heat, and simmer 30 minutes or until chestnuts are tender and mixture thickens.
Microwave chocolate in a glass bowl at HIGH 30 seconds to 1 minute or until melted, stirring once.
Remove chestnut mixture from heat. Stir in melted chocolate and rum.
Process chestnut mixture, in batches, in a food processor until smooth, stopping to scrape down sides. Cool completely.
Beat cream at medium speed with an electric mixer until soft peaks form. Gently fold whipped cream into chestnut mixture. (Mixture will be soft.) Cover and chill at least 1 hour.
Spoon mixture into prepared pastry shells. Garnish, if desired.
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