Line an 8-in. square pan with foil and grease the foil with 1 teaspoon butter. Sprinkle with half of the crushed candy; set aside.
In a microwave-safe bowl, combine chocolate chips and 2 tablespoons butter. Microwave on high for 1 minute; stir. Microwave at additional 15-second intervals, stirring until smooth. Stir in 2/3 cup milk. Carefully pour over candy layer in prepared pan. Refrigerate for 10 minutes.
Meanwhile, in a microwave-safe bowl, combine white baking chocolate and remaining butter. Microwave on high for 40-50 seconds; stir until smooth. Stir in the cream of tartar, remaining milk and candy. Spread over chocolate layer.
Refrigerate for 2 hours or until firm. Using foil, remove fudge from pan. Invert onto a cutting board; remove foil. Cut into 1-in. squares. Yield: about 1-3/4 pounds.