Preheat your oven to 350°F (175°C).
In a large mixing bowl, combine the chocolate cake mix, eggs, water, and vegetable oil. Mix until well combined.
Line a cupcake pan with cupcake liners and pour the batter into each liner, filling them about two-thirds full.
Bake in the preheated oven for 18-20 minutes or until a toothpick inserted into the center comes out clean.
Let the cupcakes cool completely on a wire rack.
Once cooled, take the white frosting and divide it into two bowls. In one bowl, add red food coloring to create a pinkish color and mix well.
Frost the top of each cupcake with the white frosting using a spatula.
Using the piping bag, pipe the pink frosting onto the cupcakes in a zigzag pattern to resemble the appearance of a brain.
Place mini marshmallows around the edges for added decoration.
For an extra touch, sprinkle chocolate chips on top or use them to create small 'veins' on the brain frosting.
Serve and enjoy your spooky Brain Halloween Cupcakes!