In a small bowl, combine butter and cocoa until smooth. In a large bowl, beat eggs and sugar. Stir in chocolate mixture. Combine flour and salt; gradually add to chocolate mixture just until blended. Stir in walnuts. Pour into a greased 13-in. x 9-in. baking pan; set aside.
For butterscotch layer, in a large bowl, cream butter and brown sugar until light and fluffy. Beat in eggs and vanilla. Combine flour and salt; gradually add to creamed mixture. Stir in walnuts.
Spoon over chocolate layer. Bake at 350° for 30-35 minutes or until brownies begin to pull away from sides of pan. Cool on a wire rack.
For frosting, in a small saucepan, combine the butter, brown sugar and milk; bring to a boil and boil for 2 minutes. Remove from the heat; gradually stir in confectioners' sugar until smooth. Quickly spread over brownies.
In a microwave, melt the chocolate chips, butterscotch chips and shortening at 70% power for 1 minute; stir. Microwave at additional 10- to 20-second intervals, stirring until smooth. Drizzle over frosting.