For ganache, combine butterscotch chips, cream, and ¼ teaspoon salt. Microwave at half (50%) power 1 minute; stir. Microwave on 50% power in 30-second intervals, stirring between each, until chips are almost melted. Stir thoroughly until mixture is smooth. Cool slightly.
In large bowl, beat butter and shortening with electric mixer. Gradually add sugar, one cup at a time, beating well on medium speed. Scrape sides and bottom of bowl often. Add 2/3 cup cooled butterscotch ganache and beat until well combined. Reserve 1-3/4 cups icing. Stir 1/4 cup mini chocolate chips into remaining icing.