Coat two 8-inch round cake pans with cooking spray and line the bottoms with parchment paper; spray the parchment. Divide the butter and salt between two separate pots and melt over medium heat. Add half the marshmallows to each pot and cook, stirring occasionally, until melted. Remove from the heat and stir 1/2 teaspoon vanilla into each pot. Stir the blue and black food coloring into one pot to tint the marshmallows dark blue. Stir 3 cups cereal into each pot until coated.
Coat your hands with cooking spray. Working quickly, divide the blue cereal mixture evenly between the two pans and form into a compact 1 1/2-inch-wide ring around the edges. Use the plain cereal mixture to fill in the center of each pan, packing it tightly. Let harden slightly, about 15 minutes, then turn out onto a cutting board. Cut each round into quarters.
Put the white candy melts in a microwave-safe bowl and microwave in 30-second intervals, stirring, until melted. Let cool slightly, then transfer to a resealable plastic bag. Snip off a corner and pipe a white mouth onto each treat. Let harden, about 15 minutes. Melt the red candy melts in the microwave. Let cool slightly, then transfer to a separate bag and snip o a corner; pipe a zigzag on top of the white candy. Let harden. Press a chocolate chip into each treat for the eye.