Place baking chips in a single layer on a small baking sheet. Freeze 30 minutes. Line a 9-in. square pan with foil; grease foil with butter. , In a large microwave-safe bowl, combine dark chocolate chips and milk. Microwave, uncovered, on high for 2 minutes; stir. Microwave in additional 30-second intervals, stirring until smooth. Stir in liqueur and salt. Add white baking chips; stir just until partially melted. Spread into prepared pan. Refrigerate 1 hour or until firm., Using foil, lift fudge out of pan. Remove foil; cut fudge into 1-in. squares. Store in an airtight container in the refrigerator.