- Preheat oven according to cake mix directions. Grease and line two 8-inch round cake pans. 
- In a bowl, whisk eggs, egg yolk, hot chocolate, and oil. Add cake mix and beat until smooth. 
- Divide batter evenly between pans and bake 28-30 minutes or until a toothpick comes out clean. 
- Cool cakes for 10 minutes in pans, then transfer to a rack to cool completely. Wrap and refrigerate if making ahead. 
- For ganache, heat heavy cream until simmering, pour over chocolate chips, let stand 2 minutes, then stir until smooth. Cool to room temperature. 
- For buttercream, beat butter until fluffy, add marshmallow fluff and vanilla, mix gently, then add powdered sugar gradually. 
- To assemble, place first cake layer on a board, spread buttercream, drizzle ganache, top with second cake layer, repeat frosting and ganache. 
- Top with mini marshmallows and toast with a kitchen torch.