Preheat the oven to 350°F and line a baking sheet with parchment paper.
In a mixing bowl, cream the butter and sugar together until light. Add the egg and vanilla, then stir until combined.
Mix in the cocoa powder, flour, baking soda, and salt until a thick dough forms. Fold in the chocolate chips.
Scoop the dough into four equal portions and place them on the prepared baking sheet. Flatten each slightly with your hand.
Bake for 10 to 12 minutes until the edges are set. Let the cookies cool on the sheet for 5 minutes, then move them to a wire rack.
Once the cookies are fully cooled, press one marshmallow upright into the center of each cookie to form a tombstone.
Sprinkle the crushed chocolate sandwich cookies around the base of each marshmallow to create the graveyard soil.