Adjust oven rack to center position and heat oven to 375°F.
Line 8-inch square baking pan with aluminum foil, making sure the foil is tucked into all corners and that at least 1 inch of foil overhangs top of pans on all sides.
Line bottom of pan with graham crackers in a single layer, breaking them if necessary to fit tightly.
Combine butter, brown sugar, and salt in small saucepan and cook over low heat, stirring constantly, until butter is melted and sugar has dissolved.
Scrape butter-sugar mixture onto graham crackers and smooth with small rubber spatula to cover crackers. Bake until caramel is bubbling. 10-12 minutes.
Remove pan from oven, sprinkle evenly with chocolate chips, and return to oven to soften chocolate, 2 minutes. Remove pan from oven and, using rubber spatula, smooth chocolate into even layer.
Sprinkle marshmallows and then nuts over chocolate. Press lightly with fingertips to adhere marshmallows and nuts to chocolate.
Cool on wire rack 30 minutes, then freeze until chocolate hardens, about 30 minutes.
Grabbing foil overhang, lift from pan onto cutting board and use knife to cut bark into 2-inch squares.
Layer pieces between sheets of waxed paper in airtight container and refrigerate for up to 1 week.