Line a large baking sheet with parchment paper. Set aside.
In a large pot, melt the butter over medium-low heat. Once melted, add the marshmallows and food coloring and stir regularly until almost fully melted. The color should be a nice red, but it'll lighten once the cereal is mixed in.*Note: Having the odd white piece of marshmallows helps to give the brains some contrast. So don't worry if they're not 100% melted.
Remove the pot from the heat and add the Rice Krispies. Mix quickly until evenly combined.
Lightly grease a ¼ cup measuring cup (round bottom preferred) with non-stick cooking spray. Fill it with the rice krispie mixture and drop a level scoop onto the prepared baking sheet. Working quickly, repeat until all the cereal is used, respraying with non-grease spray as needed (about every 3 scoops).
Shape the scoops into an oblong shape, keeping the bottom flat to resemble a brain. With the back of a butter knife, lightly press down the center to create two “brain hemispheres”.
Design as desired with red gel icing, drawing squiggles over the brains. Allow them to fully set up for about 2 hours before serving.
Notes
If you only have unsalted butter on hand, add 1/4 teaspoon of salt to the melted butter.