In a large mixing bowl, combine all-purpose flour, sugar, cocoa powder, baking soda, and salt. In a separate bowl, cream together the unsalted butter and granulated sugar until fluffy. Then mix in eggs, buttermilk, vanilla, red food coloring, and white vinegar before combining wet and dry ingredients until smooth.

Preheat your oven to 350°F (175°C). Fill cupcake liners with batter and bake for about 18-20 minutes or until a toothpick inserted comes out clean. Let them cool completely.

Using a knife or melon baller, hollow out the center of each cooled cupcake. Mix together red velvet cake crumbs and sweetened condensed milk to create the 'blood' filling.

Beat softened butter until creamy, gradually add powdered sugar and milk until achieving desired frosting consistency. Add vanilla extract and food coloring to tint.

Fill the hollowed cupcakes with the 'blood' filling, frost them with the skin-tone buttercream, and decorate with candy fangs, red gelatin marks, and drizzle with red syrup for a bloody effect.