Preheat your oven to 350°F and grease your cake pans well. Whisk together the eggs, oil, buttermilk, vinegar, vanilla, and food coloring in one bowl. In a separate bowl, whisk the flour, sugar, cocoa powder, salt, and baking soda, then combine both mixtures until smooth.
Divide the batter evenly and bake for 25 to 30 minutes, until the layers spring back when lightly touched. Let them cool in the pans for 10 minutes, then move them to a wire rack. Once completely cool, trim the domed tops flat with a serrated knife.
Fill and stack your layers with your favorite buttercream, then apply a thin crumb coat around the entire cake. Chill the cake for about 15 minutes so this coat firms up. Add a second, slightly thicker layer of frosting on top, and don't stress over making it perfectly smooth.
Finely chop your red candy melts and place them in a heatproof bowl. Warm the heavy cream in a saucepan until it's just about to simmer, then pour it over the candy melts. Stir until completely smooth and glossy.
Spoon the warm ganache onto the center of your chilled cake. Use an offset spatula to gently push it toward the edges, letting it naturally drip down the sides in uneven streaks for that classic "bloody" effect.
Pipe bat outlines onto parchment paper using melted black candy melts, then insert a small stick or wire into each one while still wet for support. Once set, peel them off, attach candy eyes with a dab of melted candy, and press them into the top of the cake at varying heights.