Special equipment: a candy thermometer
Preheat the oven to 250 degrees F. Line 2 baking sheets with silicone baking mats or wax paper.
Place the popcorn, marshmallows, chocolate chips and graham crackers in a large bowl; do not combine yet.
Combine the butter, granulated sugar, brown sugar and corn syrup in a medium saucepan fitted with a candy thermometer. Bring to a boil over medium-high heat, stirring until the butter is melted and the ingredients are completely combined, about 5 minutes.
Continue to boil the sugar, no longer stirring, until it reaches the firm-ball stage, 245 degrees F, 5 to 7 minutes. Remove the sugar from the heat and add the salt and baking soda.
Spray a rubber spatula with cooking spray. Immediately pour the sugar over the popcorn mixture and, using the greased spatula, carefully fold all the ingredients together. Spread the mixture evenly over the prepared baking sheets and bake until lightly browned and crispy and the chocolate and marshmallows are fully melted, about 20 minutes. Remove from the oven and let cool at least 1 hour in a cool dry place until the chocolate sets.