Preheat oven to 350 degrees. Coat pans with non-stick spray.
Using electric mixer, combine cake mix, eggs, hot chocolate and oil on medium speed for 2-3 minutes or until well blended.
Pour half of the batter into each of the prepared pans.
Bake cakes for 15-17 minutes each or until toothpick inserted into the center comes out clean.
Remove pans from the oven and allow to cool for 25-30 minutes. Run an knife around the edges of the cakes, flip pans over to gently extract cakes. Let cakes cool completely before frosting.
Make buttercream frosting while waiting for cakes to cool.
Fill disposable decorating bag with icing. Cut tip of icing bag about 1/2 inch.
Using biscuit cutter, cut 12-15 circles out of each cake (or more, depending on size of cutter).
Using filled decorating bag, begin to pipe icing on half of the cake circles.
After half of the cake circles have icing, top with remaining cake circles.
Using a vertical motion, pipe a dollop of icing on each cake top.
If desired, decorate top of each mini cake with mini marshmallows and sanding sugars.