Heat oven to 350℉/177℃.
Line 2 sheet pans with parchment paper. Set aside.
Combine flour, cocoa, sugar, salt, and baking powder in the bowl of a food processor fitted with the dough blade.
Pulse a few times to combine.
Add butter, a little at a time, pulsing after each addition.
Process until butter is evenly distributed through flour mixture.
Add egg yolk.
Pulse to combine.
Add cream.
Process until mixture becomes a smooth dough.
Remove from food processor.
Roll out on an un-floured non-stick surface (marble, parchment paper, silicone mat) to ⅛-inch thick.
Use a ghost cookie cutter to cut out shapes, re-rolling as necessary.
Place half of ghosts on prepared pans.
Spoon marshmallow fluff into a pastry bag, or zip top bag, and snip the tip off.
Pipe marshmallow onto ghost, staying away from the edges.
Place other half of ghosts on top of marshmallow fluff.
Press around edges to seal. (Use a fork to insure you have a good seal.)
Bake for 10 minutes.