Step 4
In small microwavable bowl, microwave chocolate chips and butterscotch chips uncovered on High 60 seconds; stir halfway through. Continue to microwave in 15-second increments, stirring after each, until chips can be stirred smooth; cool 10 minutes. In large bowl, beat 1 1/2 cups softened butter and melted chips with electric mixer on medium speed until blended. On low speed, beat in powdered sugar, milk and vanilla, scraping side of bowl occasionally, until smooth and creamy.