Preheat oven to 325 degrees F. Line a 13 x 9 inch baking pan with foil extending over the sides. Spray lightly with nonstick cooking spray. Set aside.: Warm, slightly sweet air from the oven signals the right environment for the crust to crisp while remaining tender, and lining the pan with foil makes lifting the bars out effortless once cooled. You should feel the kitchen warm as the oven approaches temperature, and the foil edges will glint when you check the pan. Using a cold pan or skipping the spray can cause the crust to stick, making it hard to remove the bars cleanly. A common mistake at this stage is not pressing the foil into corners, which creates gaps and leads to uneven baking along the edges.
In a large bowl, combine graham cracker crumbs, melted butter and brown sugar. Stir until evenly combined then press mixture firmly and evenly into the bottom of the prepared pan. Bake for 12 minutes. Cool crust in pan on a wire rack for 5 minutes.: The aroma of toasted crumbs and warm butter becomes noticeable as you blend the dry and wet components, and pressing firmly creates a compact, cohesive base that will hold the gooey layers. You want the crust to be even in thickness so it bakes uniformly and supports the toppings; use flat bottom of a measuring cup to pack it. If you under-press, the crust can crumble when sliced, and over-pressing can make it too dense. Watch for a faint golden rim around the pan to know it is ready.
Meanwhile, microwave caramels and heavy cream together in a medium bowl until smooth, stirring every 30 seconds until melted, about 2 minutes. Pour caramel mixture over crust and spread evenly.: When melting the caramel with heavy cream , you will see a glossy, velvety sauce form, and its warm scent will remind you of toffee. Stirring frequently yields a silky texture and prevents hot spots that scorch. Pouring while still fluid helps the caramel sink into the crust slightly, creating a bonded center. A typical error is overheating the caramel, which can seize and become grainy, so use short bursts and stir between intervals.
Sprinkle caramel with marshmallows, nuts and chocolate, slightly pressing them into the caramel.: As you scatter the tiny marshmallows , chopped nuts , and chocolate chips , you should hear a soft pat as pieces meet the warm caramel, and the contrast of matte marshmallows and shiny chocolate looks inviting. Slightly pressing them in secures them so they do not slide during baking, and it helps the layers fuse. Avoid piling items too densely, which can prevent even browning and make cutting messy later.
Bake at 325 degrees F for 12 minutes then broil just until marshmallows begin to brown. Cool in pan on a wire rack. Use foil to lift uncut bars out of pan and onto a cutting board. Cut into bars and enjoy!: The oven time lets the caramel set and the chocolate soften, while a quick broil gives the marshmallows a toasty top, producing that campfire aroma and little browned peaks. Watch closely under the broiler; marshmallows can go from golden to charred in seconds. Once cooled on a wire rack, lifting the foil brings the whole slab out intact, and the visual of clean edges makes cutting satisfying. Cutting while overly warm can smear the layers, so let the pan rest until the caramel firms slightly for neat bars.
