Melt chocolate in a heavy saucepan over low heat, stirring until smooth. Meanwhile, beat whipping cream at medium speed with an electric beater until soft peaks form. Gradually add melted chocolate, and beat 15 minutes. Cover and chill 8 hours.
Working quickly, shape half of chocolate mixture into 1-inch balls. Place balls on a wax paper-lined rimmed baking sheet. (Keep remaining half of chocolate mixture chilled.) Repeat procedure with remaining chocolate mixture. Roll balls in walnuts until completely coated. Store in an airtight container in refrigerator.