Line bottom and sides of an 8-inch square pan with parchment paper, allowing 2 to 3 inches to extend over sides.
Microwave butter in a large microwave-safe glass bowl at HIGH for 30-second intervals until melted. Gently stir in powdered sugar and next 3 ingredients. (Mixture will be lumpy.) Microwave 30 seconds; add vanilla.
Beat mixture at medium-low speed with an electric mixer until well blended and smooth. Pour into prepared pan, spreading to edges of pan.
Microwave white chocolate and whipping cream in a small microwave-safe glass bowl at HIGH until white chocolate is melted (about 30 seconds to 1 minute), stirring at 30-second intervals. Stir until mixture is smooth. Let stand 1 to 3 minutes or until slightly thickened. Spoon mixture over fudge in pan, swirling with a paring knife. Cover and chill until firm (about 2 hours).
Note: We tested in an 1,100-watt and a 1,250-watt microwave oven. Cook times will vary depending on your microwave wattage; be sure to follow the descriptions in the recipe for best results.