Preheat your oven to 350°, and line two baking sheets with parchment paper.
Beat together butter and brown sugar until light and fluffy. Beat in egg and vanilla extract until well combined.
Sift flour, cocoa powder, baking soda and salt.
Add half the dry sifted ingredients to the wet ingredients and beat for 30 seconds. Then add half the buttermilk and continue to beat. Repeat these steps until well combined – you should have a thick sticky batter.
Using a 2 tablespoon cookie scoop, mound batter onto prepared baking sheets about 3” apart (if you don’t have a cookie scoop, mound two tablespoons of the batter).
Bake for 10-12 minutes. Cool completely.
Meanwhile, in a large mixing bowl combine frosting and orange food colouring and mix. You might need more orange food colouring if you want a deeper colour.
To assemble, spread about 2 tablespoons of coloured frosting onto the flat surface of one of the cookies. Top with another cookie. Repeat until all whoopie pies are complete.
Place miniature marshmallows into a microwave safe bowl and microwave at 15 second intervals until melted. Using a fork, mix the marshmallows and let sit for a minute to let cool. Using your fingers take a bit of the marshmallow mixture and stretch it over the whoopie pies to create a cobweb.