Preheat oven to 350 F. Line two baking sheets with parchment paper or silicone baking mats.
Add butter, brown sugar, baking powder, baking soda, salt and vanilla to the bowl of a stand mixer and mix until smooth. Add egg, mixing until smooth. Add cocoa powder and stir to incorporate.
Alternate adding flour and milk, scraping down sides of bowl after each addition, and mix until smooth. Batter should resemble a thick and fluffy cake batter.
Dollop six, 1/4 cup scoops of batter onto each baking sheet, leaving space between each cake, as batter will spread while baking. Bake for 15-17 minutes, until the center of each cake is firm when gently tapped. Allow to cool for 10 minutes, then move cakes to a wire rack and cool completely. (For smaller cakes use less batter and decrease baking time by about 3 minutes.)
Make the syrup. Combine cherry syrup and sugar in a small saucepan. Bring to a boil until sugar has dissolved and mixture has thickened slightly. Remove from heat and stir in extracts and (optional) liquor. Allow to cool before brushing the undersides of each cake with syrup.
Make the filling. Add shortening, 1 cup confectioners' sugar, and marshmallow fluff/creme to the bowl of a stand mixer. Mix until smooth, then add salt water and vanilla and mix again until smooth. If the filling is too loose, add 1/3 cup more confectioners' sugar to stiffen.
Assemble the pies. Fill a piping bag or use a small offset spatula to apply a thick layer of frosting to the underside of six cakes. Gently press 6-7 cherry halves into the filling before placing another cake, underside-in, on top, creating a sandwich. Repeat until all 6 cakes are assembled.
To make Vampire Bites, poke two holes in the top of each cake and drizzle red gel icing from each hole.