Melt the marshmallows and butter over very low heat, stirring occasionally, about 5 minutes. Remove the pan from the heat, then squeeze in two drops of red food coloring. Slightly stir, allowing marble effect. Add the popcorn and mix until covered with the marshmallow.
Either with lightly oiled hands, latex gloves, or between wax paper, shape the popcorn into eight equal sized balls. Place on sheet of wax or parchment, then press down slightly to achieve more of a brain shape. Squeeze another drop or two of red food coloring on the top of each popcorn ball, then use a paintbrush to paint a solid red line down the center. Paint on additional "blood" as desired. Allow to cool completely before serving.