It may even be better without it. Most Mississippi Mud Pie recipes don't call for essence. Is there another form of chocolate or cocoa in the recipe?

The chocolate essence would only be there to add chocolate flavor, and wouldn't affect how well it sets up, etc. So as long as you have plenty of other good chocolate in there, it should be delicious. If you can post a link to the recipe on this thread, we could help suggest substitutions.