In a bowl, combine the powdered sugar, cocoa and salt. Add the coffee or orange juice or substitute with 1 tbsp of cold water if you do not want to use coffee or orange juice. Add the mascarpone and with an electric mixer, beat on low speed till everything is well mixed and looking a little fluffy. Now add the cream, and slowly increase the speed and continue beating till the mixture forms soft peaks. Do not over beat.
Scrape the mousse into a bowl and refrigerate for at least 4 hours, but overnight is preferable. Serve as dessert or use to make other desserts.
Puff pastry involves using a lot of butter and entails quite some effort though this would probably come down with practice. So it is criminal to throw out the scraps leftover from making vol-au-vents. So I just piled my scraps one on top of another and rolled out the pastry to make palmiers. I used brown sugar here and baked them at 230C for about 18 to 20 minutes.
Where I live, summer is quite warm and the cooler months (July through January) are still too warm to comfortably make puff pastry. Having said that, a bit of practice with rolling and folding the dough is all it takes to make good puff pastry at home. Just ensure that you refrigerate the dough the moment you feel that the butter in the dough is becoming too soft to handle. Let the dough stay in the fridge until the dough has chilled enough to handle and this might take longer than mentioned.
We enjoyed the vol-au-vents and the mille feuille as, in both, the contrast of buttery crispness with the soft filling was interesting as always.
Puff pastry with savoury filling is something we have eaten a lot in the form of "puffs" (a turnover with a variety of savoury filling which is quite popular here) and enjoy very much.
So while we liked the savoury vol-au-vent, the sweet one wasn't quite to our liking. The puff pastry was great and the mousse was fantastic but they didn't really work well together for us. Luckily, I had sandwiched only a few, so we ended up eating the puff pastry and the mousse separately.