Preheat oven to 350°F (175°C). Grease and line two 9-inch round cake pans with parchment paper.
In a large bowl, cream together butter and sugar until light and fluffy.
Add eggs one at a time, beating well after each addition.
Mix in vanilla extract.
In a separate bowl, whisk together flour, baking powder, and salt.
Alternately add dry ingredients and milk to the butter mixture, mixing until just combined.
Pour batter evenly into prepared pans.
Bake for 30–35 minutes, or until a toothpick inserted comes out clean.
Let cakes cool in the pans for 10 minutes, then transfer to wire rack to cool completely.
While cakes cool, make frosting: beat 1 cup butter, 3 cups powdered sugar, 2 tablespoons milk, and a few drops of pink food coloring until smooth.
Once cool, trim and carve cakes into a domed brain shape using a serrated knife.
Place cake on a tray. Spread a layer of frosting over entire surface.
Fill piping bag with pink frosting and pipe squiggly brain folds across the top and sides of the cake.
Add drips of red gel or raspberry jam between folds for a gory effect.
Chill in refrigerator until ready to serve.