How to Make It
Preheat the oven to 350°. Spread the walnuts in a pie plate and toast until golden and fragrant, about 8 minutes. Let cool then coarsely chop.
In a large bowl, mix the melted butter with the graham cracker crumbs until evenly moistened. Press the crumbs onto the bottom of a 15-by-17-inch baking pan. Scatter the semisweet chocolate chips on top, followed by the butterscotch chips, white chocolate chips, toasted walnuts and coconut. Scrape the condensed milk into a microwave-safe container and heat it at high power for 30 seconds, until just fluid. Drizzle the milk evenly over the coconut.
Bake the bars in the center of the oven for about 30 minutes, until the coconut is toasted; rotate the pan once for even cooking. Transfer the pan to a rack and let the bars cool completely, at least 4 hours. Cut the bars lengthwise into 6 strips and crosswise into 6 strips to make 36 bars.