Special equipment: a cupcake corer; a pastry bag fitted with a medium round tip
For the cupcakes: Preheat the oven to 350 degrees F. Line a 12-cup muffin tin with paper liners.
In the bowl of a stand mixer fitted with the paddle attachment, cream the butter and sugar for about 2 minutes, until light and fluffy. Add the eggs one at a time, beating after each addition. Beat in the egg white, scraping the sides of the bowl as you go. Add the vanilla and beat on medium-high speed for about 1 minute.
In a separate medium bowl, whisk the flour, baking powder and salt. Mix the flour mixture and the milk into the egg mixture in three parts¿flour-milk-flour, taking care to scrape down the sides as you go, and incorporating each addition well. Mix until smooth.
Divide the batter evenly between the muffin cups using an ice-cream scoop--the batter should come about three-quarters of the way up the sides. Bake until a toothpick inserted in the center comes out clean, 15 to 16 minutes.
Let the cupcakes cool in the pans on a wire rack for 10 to 15 minutes. Then remove from the pan to the rack to cool completely.
For the filling and decoration: Use a cupcake corer to hollow out the center of the cupcakes, saving the tops; then fill with a couple tablespoons of candy goo. Slice the top off of the plug you removed from the cupcake and place back in the hole to seal in the filling. Use a couple of drops of red food coloring and the smallest amount of yellow and black to color your frosting. Frost the tops of cupcakes with a thin layer of buttercream. Place the remaining buttercream into a pastry bag fitting with a medium round tip. Pipe squiggles around the top of the cupcakes, creating two hemispheres of the brain. Thin the red piping gel with a teaspoon of water and use a spoon to drizzle over top of the brain to get into crevices.
In the bowl of a stand mixer fitted with the whisk attachment, combine all the ingredients. Whip the mixture on low speed for a while. When the sugar is incorporated, turn the mixer speed up and whip until the buttercream is smooth and looks right to you. If it's too cold, persuade it with a blowtorch if you have one. If it's too wet, add a bit more powdered sugar. If it's too stiff, add a bit more butter. Be sure to scrape the sides of the bowl once or twice to make sure everything is combined.
Use immediately, or store in an airtight container at room temperature for 24 hours or in the fridge for up to 2 weeks. Rewhip cold buttercream before using it.
Adapted from "Duff Bakes: Think and Bake Like a Pro at Home" by Duff Goldman © William Morrow 2015. Provided courtesy of Duff Goldman. All rights reserved.