In large bowl, stir together flour, baking powder and salt.
In large bowl, beat butter and sugar with electric mixer until light and fluffy. Add eggs one at a time, mixing well after each. Add vanilla; mix well. Add flour mixture alternately with milk, beating until just combined. Divide evenly into pans.
Bake 19-22 minutes, or until toothpick inserted in center of cakes comes out clean. Cool in pan on cooling grid 5 minutes. Remove from pans; cool completely on grid.
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.
To assemble, cut the crowns off of each cake layer. Place first layer on serving plate. Spread ¼ of icing with chocolate chips to edge; top with second layer. Continue with remaining cake layers and icing. Ice top layer and sides of cake with reserved 1-3/4 cups plain icing. Refrigerate until icing is firm, about 20-30 minutes. Microwave remaining 2/3 cup butterscotch ganache on 50% power for 15-20 seconds, or until just warm. Pour over top of chilled cake, spreading to edges and allowing to run down sides. Sprinkle additional mini chocolate chips on top. Refrigerate at least 5 minutes before cutting.