Lightly grease (or line with parchment) a 9” square pan. , Preheat the oven to 350°F and place a rack in the top third. , In a medium saucepan set over medium heat, melt and stir together the butter, brown sugar, and syrup, cooking until the sugar has dissolved. , Stir in the oats, flour, salt, graham cracker crumbs, and cinnamon. , Press about ¾ of the mixture (about 500g) evenly into the prepared pan. Reserve a generous cup (about 170g) for the topping. , Bake the crust for 15 to 20 minutes. , Remove the crust from the oven and sprinkle the chocolate chips evenly over the hot crust. Wait 5 minutes, then spread the melted chocolate evenly over the crust. (A small offset spatula is a helpful tool here.) , Sprinkle the marshmallows over the chocolate, then top the marshmallows with the remaining crust mixture; this is the topping. , Bake for 10 to 15 minutes, until the marshmallows are puffed. , Turn the oven to broil and broil the bars for 1 to 2 minutes until the marshmallows are toasted to your liking. This happens quickly, so be sure to keep a close eye on the bars to prevent them from burning. , Remove the bars from the oven and cool completely on a wire rack (in the pan) before cutting. , Serve bars at room temperature or reheat for a nostalgic, warm-from-the-campfire s’more experience. Bars can be reheated for 10 seconds in the microwave or wrapped in foil and warmed in a 350°F oven. , Store leftover bars, well wrapped, at room temperature for several days; freeze for longer storage.