Begin by preheating your oven to 350 degrees F. Prepare your baking mold by spraying it with baking spray. Dust lightly with cocoa powder. Set aside.
In a medium bowl, whisk together the flour, cocoa powder, baking soda, and cinnamon; set aside.
With a mixer, beat the butter and sugar for 3-5 minutes or until light and fluffy. Add the eggs, one at a time, mixing well after each addition. Scrape the sides of the bowl as needed.
Add the flour mixture to the butter mixture, alternating with the buttermilk.
Fill the molds about 60% full. Bake for 13-15 minutes or until a toothpick inserted near the center comes about clean. Cool on a wire rack for 5 minutes. Remove the cakes from the mold and allow them to continue to cool. Once completely cool, use a serrated knife to cut away the domed back of the cake, if desired.
In the meantime, prepare the marshmallow creme filling. Place the marshmallow creme and butter in a mixing bowl. Beat on medium-high until smooth. Add the confectioners' sugar, cocoa powder, vanilla extract, salt, and red food dye. Mix well until light and fluffy. Transfer to a piping bag fitted with a small round tip.
Prepare the confectioners' sugar glaze by whisking together the confectioners' sugar, milk, and vanilla until smooth. For a thinner glaze, use additional milk. Set aside.
To assemble, use a large wood skewer to create a tunnel from the bottom to the top of each tombstone. Once inserted, wiggle the wood skewer to create room for the filling. Pipe the marshmallow creme filling into each cake. Carefully wipe away any excess.
Use a pastry brush to add the confectioners' sugar glaze to all sides of the snacks cakes.
Serve immediately or store in an airtight container. Before serving, use any leftover red marshmallow creme filling to decorate the outside of the tombstones.