Heat your oven to 350°F and line a 12-cup muffin tin with paper liners.
In a big bowl, whisk together the flour, sugar, cocoa, baking powder, baking soda, and salt. Stir in the buttermilk, oil, egg, and vanilla until smooth. Finally, mix in the boiling water until fully combined.
Divide the batter evenly among the liners, filling each about two-thirds full.
Bake for 18 to 20 minutes, until a toothpick inserted in the center comes out clean. Let the cupcakes cool completely, about 1 hour.
For the frosting, beat the powdered sugar, butter, vanilla, and salt until light and fluffy. Add the green food coloring and beat until the color is even. Transfer to a piping bag fitted with a small round tip.
Pipe a small mound on the center of each cupcake. Pipe two straight vertical lines side by side down the middle, then pipe squiggly lines on either side to look like brain folds.
Drizzle red decorating gel over each cupcake, letting it settle into the creases for a bloody effect.
Notes
Nutritional Information per serving: Calories: 480, Total Carbs: 62g, Net Carbs: 60g, Fats: 26g, Protein: 4g.