Line a 9-inch by 13-inch pan with foil, leaving a 2-inch overhang on two sides. Lightly coat the foil and a wooden spoon with cooking spray.
Melt the butter in a medium saucepan over medium heat. Add the marshmallows, honey, vanilla and salt. Stir with the prepared wooden spoon until the marshmallows have completely melted, 4 to 5 minutes. Add the rice cereal and puffed millet and stir until the mixture is fully combined. Transfer the mixture to the prepared pan and press into an even layer. Let sit at room temperature until firm, about 20 minutes.
Using the overhanging foil as handles, lift the treats out of the pan and invert onto a cutting board. Cut into 15 pieces. The treats can be refrigerated in an airtight container for up to a week. Let sit at room temperature for 30 minutes before serving for a gooier texture.
The cereal-and-marshmallow mixture will be sticky. Try wetting your hands before patting it down.
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