How to Make It
Line an 8-inch square baking pan with parchment paper, leaving a 2-inch overhang on all sides. Coat with cooking spray; set aside. Stir together brown rice cereal, puffed grains, pepitas, sunflower seed kernels, and poppy seeds in a large bowl; set aside.
Combine marshmallows and butter in a medium saucepan. Cook over medium, stirring constantly, until mixture is melted and smooth, 4 to 5 minutes. Immediately pour marshmallow mixture over puffed grain mixture in bowl; quickly stir until cereal, grains, and seeds are evenly coated. Transfer mixture to prepared pan. Grease hands with cooking spray; press mixture into an even layer in pan, smoothing top. Sprinkle with flaky sea salt. Let stand at room temperature at least 30 minutes or up to 2 hours.
Using parchment paper overhang as handles, remove cereal mixture from pan. Using a serrated knife, cut into 16 (2-inch) squares.
Find puffed grains at health food stores and on amazon.com.