The dessert pictured above definitely, definitely has gelatin in it. That will change the mouth feel on the mousse slightly, but it might be what you are going for and will provide some of the stability that you are looking for. To have it hold a form like this you will need a recipe that requires gelatin; I would recommend gelatin sheets if you can find them at a local baking/cooking supply store or order them online.
The fluffiness will also be determined by several things. How were the whites whipped (too much, too little) or how was the cream whipped (again too much, too little)? There are recipes that call for both. How you incorporate all of the ingredients into one another really matters and in what order, this is called tempering. You want to fold until the mixtures are just combined; no more.
Temperature is of key importance for all of the ingredients - some you want very cold while others (ie chocolate) you want to be room temperature but not too cool or it will seize up the mixture and the mousse will taste grainy.
Also chocolate mousse texture can definitely be affected by the percentage and quality of chocolate you are using. If is is of a higher percentage it can create a denser mousse and deflate the whites and cream more easily.
Real chocolate (cocoa solids, cocoa powder, and sugar) is not the best thing for what you want. If properly handled, it can give you a great tasting glaze - but you don't have the conditions to handle it properly (slow cooling), so you will run into problems with it (bad texture, sweating, bad looks/dull). Also, it won't be a really snappy shell. And at this temperature, the good flavor is not very noticeable anyway.
What you want to use is cocoa-containing fat glaze. This is the stuff which commercial "chocolate" coated ice cream like the Magnum brand uses. It is sold in supermarkets as "chocolate cake glaze" and similar names, but check the ingredient list to be sure. It should consist of cocoa solids and vegetable fat, possibly some emulsifiers too.
You should only melt your glaze, preferably have a low-temperature water bath at your booth (not full boil), and hold the glaze there for dipping. Don't add milk at all. Milk is a bad choice for chocolate covering anyway. Cream is used for making cake icings called ganache (mixed with real chocolate, not this glaze stuff), but it makes a soft, smeary icing, not a hard, snappy shell. Use the glaze the way it is, without adding anything.
This glaze is quite hard at room temperature already. A thin layer of it on a frozen banana should go to below room temperature almost instantly. I think you will have your shell without any special preparation. But I have never done it this way, so I would advise you to experiment. If my hunch is wrong, there still isn't anything you can add - this stuff doesn't harden because of anything added, it hardens because at room temperature, it is a hard solid. So your only way to speed hardening is to return the banana to the freezer for a short time. But then you are likely to get a condensation problem, so keep it as short as possible.
Best Answer
I think it is more likely that the pastry chef employed a piece of food grade acetate (the link is a sample vendor).
They would have piped the pattern of tempered white chocolate onto the acetate sheet, then rolled it into a cylinder (chocolate on the inside) taping or clamping it shut to set. Once thoroughly chilled, they would have very, very carefully peeled the sheet from the chocolate.
This is a very advanced technique, especially with white chocolate, which is persnickety and tough to temper, and often not as solid as dark chocolate. Good luck.