For buttercream, beat shortening and butter in a large bowl with electric mixer until light and fluffy. Gradually add sugar, one cup at a time, beating well on medium speed. Scrape sides and bottom of bowl often. When all sugar has been mixed in, icing will appear dry. Slowly add milk; beat at medium speed until light and fluffy. Add melted candy and beat until well combined, scraping bottom and sides of bowl often. Spread buttercream over brownies. Remove with parchment; cut and serve.