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Lightly grease (or line with parchment) a 9” square pan.
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Preheat the oven to 350°F and place a rack in the top third.
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In a medium saucepan set over medium heat, melt and stir together the butter, brown sugar, and syrup, cooking until the sugar has dissolved.
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Stir in the oats, flour, salt, graham cracker crumbs, and cinnamon.
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Press about 3/4 of the mixture (about 500g) evenly into the prepared pan. Reserve a generous cup (about 170g) for the topping.
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Bake the crust for 15 to 20 minutes.
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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.)
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Sprinkle the marshmallows over the chocolate, then top the marshmallows with the remaining crust mixture; this is the topping.
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Bake for 10 to 15 minutes, until the marshmallows are puffed.
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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.
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Remove the bars from the oven and cool completely on a wire rack (in the pan) before cutting.
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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.
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Store leftover bars, well wrapped, at room temperature for several days; freeze for longer storage.