Hi Stacey.
Yes, you absolutely can add flavourings/essences, add them towards the end of the whipping. Marshmallow should dry out when left out in the air but it’s generally sticky that’s why they have a fine sugary coating. So with this slice in particular, I would slice it with a hot and dry, sharp knife and then leave the pieces out in the air until it has dried off a little. You can if you like, lightly dust the top with icing sugar. I hope this helps. 🙂