Adjust oven rack to center position and heat oven to 375 degrees. Line 8 inch square baking pan with heavy duty aluminum foil, making sure that foil is tucked into all corners and that at least 1 inch of foil overhangs top of pan on all sides. In single layer, line bottom of pan with graham crackers, 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 mixture onto graham crackers and smooth with small spatula so that mixture covers crackers completely. Bake until caramel is bubbling, about 10 minutes.
Remove pan from oven, sprinkle with chocolate chips, and return to oven to soften chocolate, 1 to 2 minutes. Remove from oven, and using spatula, smooth chocolate into even layer.
Sprinkle marshmallows and then nuts over chocolate. Press lightly with fingertips to adhere to chocolate. Cool on wire rack 30 minutes, then freeze until chocolate hardens, 30 minutes.
Grabbing overhang, lift foil from pan onto cutting board and use sharp chef's knife to cut bark into 2 inch squares.
Will keep in an airtight container in refrigerator up to 1 week.