Preheat oven to 300ºF. Grease a 1½-quart ovenproof glass bowl.
In a medium bowl, whisk together flour, cocoa powder, baking soda, and salt.
In a large bowl, with an electric mixer, beat butter until creamy. Add sugar and beat until light and fluffy, 3 to 5 minutes.
In a small bowl, whisk together eggs and vanilla. In another small bowl, whisk together sour cream and milk. Pour egg mixture into butter-sugar mixture and beat on medium speed for 2 minutes.
On a low speed, alternate adding flour mixture and sour cream mixture to butter-egg mixture, beginning and ending with flour and adding the food coloring with the first addition of liquid.
Pour the batter into the greased glass bowl until it is two-thirds full. You will have a little leftover batter.
Slow-bake until a cake tester inserted in the center comes out clean, about 1 hour and 30 minutes.
Let the cake cool and then remove it from the bowl. Level the top with a large knife and place the cake, flat side down, on a cake plate.
Use an offset spatula to frost the cake with a very thin layer of cream cheese frosting.
1. In a small sauce pan, whisk all the ingredients except the gel color and bring to a boil over medium heat.
Whisk in the gel color until well combined and cool to use.
Pour marshmallows into a microwave-safe bowl and then stir in water to evenly coat the marshmallows.
Microwave for 30 seconds and then stir. Repeat this process two more times, or until the mixture is smooth.
Sift 3 cups of powdered sugar into a large bowl and make a hole in the center. Pour the melted marshmallow mixture onto the powdered sugar. Sift the rest of the powdered sugar on top of the marshmallows.
Oil your hands to prevent the marshmallows from sticking to you. Knead in the sugar until you have the consistency of soft taffy and the fondant no longer sticks to your hands.
Time to decorate!
Roll out the fondant into ½-inch thick ropes and arrange them on the cake to look like a brain.
Use a paintbrush to brush the raspberry blood over the fondant.
TaDa! This brain cake will make the perfect creepy treat for your Halloween get-together!