Preheat the oven to 350°F. Generously grease a large silicone skull mold (or two molds if needed) with nonstick baking spray.

In a large bowl whisk together the flour, sugar, unsweetened cocoa powder, black cocoa powder, baking soda, baking powder, and salt. Set aside.

In a separate bowl add the eggs and sugar and mix until creamy. Add the buttermilk (or sour cream), vegetable oil, and vanilla. Mix until combined.

Slowly pour in the hot coffee while mixing on low speed. The batter will be thin.

Fill each skull cavity about two-thirds full, leaving room for the cake to rise. Depending on the size of your mold, you may have 8-10 skulls. **If you only have one mold it’s ok to leave the rest of the batter on the counter until ready to fill again after baking the first set.

Bake for 40-55 minutes (depending on the depth of your mold), or until a toothpick inserted into the center comes out with a few moist crumbs. Baking time will vary depending on the depth of your mold.

Let the cakes cool in the mold for about 20 minutes, then carefully remove and transfer to a wire rack. Refrigerate until completely chilled, at least 2 hours.

While the cakes cool, prepare the marshmallow filling. Beat the butter until smooth and creamy. Add about three-quarters of the marshmallow fluff and beat until light and fluffy. Mix in the vanilla, powdered sugar, salt, and milk until smooth. Add additional milk, 1 teaspoon at a time, if needed for a pipeable consistency.

If using ganache, prepare the ganache by placing the chopped chocolate in a heatproof bowl. Heat the heavy cream in a saucepan until it just begins to simmer. Pour over the chocolate and let sit for 2 to 3 minutes. Stir until smooth. Refrigerate for about 2 hours, stirring occasionally, until thick enough to spread or pipe. Do not microwave.

Turn each chilled skull over so the flat back is facing up. Transfer the marshmallow crème filling to a piping bag and pipe the filling into the back of each hollowed skull.

Pour chocolate ganache over the skull before serving.

For a classic Hostess-inspired finish, pipe a small marshmallow squiggle across the mouth of each skull after the ganache has set.

Refrigerate until ready to serve. Store covered in the refrigerator for up to 4 days.