In a small heavy saucepan melt chocolate pieces over low heat, stirring constantly. Remove from heat. Stir in walnuts, sour cream, brandy or brandy extract, and 1/4 teaspoon ground nutmeg. Transfer mixture to a bowl. Cover and chill at least 2 hours or until chocolate mixture is firm enough to shape into balls.
Line a baking sheet with parchment or waxed paper; set aside. In a small bowl combine powdered sugar, cinnamon, and 1/4 teaspoon ground nutmeg; set aside. Shape chilled truffle mixture into 3/4-inch balls. Roll each ball in the powdered sugar mixture and place on prepared baking sheet. Cover and chill until firm. Store truffles tightly covered in the refrigerator for up to 2 weeks. Let stand at room temperature about 30 minutes before serving. Makes about 30 truffles.