Gather all ingredients. Preheat the oven to 350 degrees F (180 degrees C). Coat an 8 1/2x4 1/2-inch baking pan with baking spray; line with parchment paper, leaving a 2-inch overhang on long sides. Spray parchment paper with baking spray.
Spoon ice cream into a bowl; let sit at room temperature until melted but still cool to the touch, about 30 minutes.
Stir in flour and cocoa powder until just a few streaks remain, then fold in 1/2 cup marshmallows and 1/2 cup chocolate chips. The batter will be thick.
Scrape batter into prepared pan and spread into an even layer. Sprinkle top with remaining 1/2 cup marshmallows and 2 tablespoons chocolate chips.
Bake in the preheated oven until a wooden pick inserted in center comes out clean, 30 to 35 minutes, loosely tenting with aluminum foil during the last 5 minutes of bake time if marshmallows are getting too dark. Let cool in pan on a wire rack slightly, about 10 minutes. Remove from pan and let cool completely on wire rack, about 1 hour, before slicing and serving. Recipe developed by Nicole Hopper
