Make the cupcakes: heat oven to 350°F. Line two 12-cup muffin tins with paper liners. Whisk together the flour, cocoa, baking powder, baking soda, and salt in a medium bowl.
Beat the butter and sugar in a large bowl with an electric mixer on medium-high until light and fluffy, 2 to 3 minutes. Beat in the eggs one at a time, then the vanilla, scraping down the sides of the bowl as necessary.
Reduce mixer speed to low and add the dry ingredients and milk alternately, beginning and ending with the dry ingredients and mixing well between additions. Mix until just combined.
Divide the batter among the prepared muffin tins and bake until a toothpick inserted in the center of a cupcake comes out clean, 20 to 25 minutes. Cool in the tin for 10 minutes; transfer to a wire rack to cool completely.
Make the frosting: beat the marshmallow fluff, butter, confectioner’s sugar, food coloring, and salt in a large bowl with an electric mixer until light and fluffy, 3 to 4 minutes. Frost the cupcakes, dividing evenly. For each Frankenstein cupcake: sprinkle ¼ teaspoon black sanding sugar across the top of the cupcake for the hair. Place 1 piece of the licorice underneath the hair to make the eyebrows. Place two yellow dots under the licorice to make the eyes. Use 8 black sprinkles below the eyes to make the mouth. Use 4 black sprinkle next to the eyes to make the scar. Insert the pretzel rods into the sides near the bottom to make the electrodes. For each The Bride cupcake: use ¼ teaspoon white sanding sugar to make two 1-inch stripes of grey hair 1-inch in from the sides of the top of the cupcake. Use ½ teaspoon black sugar to fill in around the grey hair. Place two yellow candy dots under the hair to make the eyes. Use 4 black sprinkles above the eyes to make the eyebrows. Use 8 black sprinkles below the eyes to make the mouth.