Lightly coat a baking sheet and a rubber spatula with cooking spray. Cook 1/2 cup of the marshmallows, the sugar, corn syrup, butter, cocoa powder, 1 tablespoon water and 1/4 teaspoon salt in a large pot over medium heat, stirring constantly with a rubber spatula, until the mixture melts completely, 3 to 5 minutes. Remove the pot from the heat, and stir in the vanilla. Add the popcorn, pretzels, graham crackers and the remaining 1/2 cup marshmallows, and stir with the rubber spatula until completely combined.
Coat the inside of a 1-cup dry measuring cup with cooking spray. Scoop up a heaping cup of the popcorn mixture, and unmold it onto the prepared baking sheet. Repeat for a total of 6 mounds on the baking sheet. Spray your hands generously with the cooking spray. Press each mound firmly with both hands to form a tight popcorn ball. Let them sit at room temperature for 20 minutes to fully set. Store in airtight containers for up to 2 days (time may vary depending on humidity levels).
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