Prepare an ice bath by filling a large bowl halfway with cold water and ice cubes. Grab a smaller bowl that fits inside the ice bath and set it nearby.
Place the chocolate and water into a small, heavy-bottomed saucepan and place over low heat. Whisk until chocolate is melted and the mixture is smooth.
Pour the chocolate mixture into the smaller bowl you set aside earlier, then place this bowl into the ice bath. Make sure not to get any of the ice water into your chocolate. Now just whisk away until the chocolate has thickened slightly. It'll take a while to thicken, but once it starts thickening, things will start happening fast. It might take up to 10 minutes if you're whisking by hand, but just a few minutes if you're going with the electric whisk.
Take care not to over-whisk, as the mousse will turn grainy. If this happens, just transfer the chocolate back to the pan and re-melt until smooth. Then pour back into the bowl, set it over the ice bath again, and re-whisk, keeping a close eye on the consistency of the mousse.
Immediately pour or scoop into serving glasses and set aside.
For the whipped cream:
Before beginning, ensure you've chilled your whisking tools for at least 15 minutes, as this will speed up the process. Add all ingredients into a mixing bowl, and use your preferred method to whisk the cream until stiff peaks form. Take care not to over-whisk or the cream will turn grainy. If this happens, you'll need to start over from scratch.
Scoop the whipped cream into a piping bag fitted with a large round tip and pipe a mound of whipped cream on top of the mousse.
Sprinkle the whipped cream with chopped pistachios and carefully place some strands of pashmak next to the cream.
Once you add the pashmak, serve immediately, as the pashmak will quickly start to melt into the cream and mousse.
Use chocolate with 70% cocoa solids, and choose a brand of chocolate you like eating plain. Be sure to use real rosewater, and not the cheap stuff you find at the average supermarket.