01 - Heat a small skillet over medium heat. Add pistachios and toast for 2-3 minutes, stirring frequently. Pour in honey and stir until nuts are evenly coated. Continue cooking for 1 minute, then spread mixture onto parchment paper to cool completely.
02 - Set a heatproof bowl over a saucepan of simmering water, ensuring the bowl does not touch the water. Add chopped chocolate and butter, stirring constantly until melted and smooth. Remove from heat and let cool for 5 minutes.
03 - Whisk egg yolks, granulated sugar, cardamom, and salt into the warm chocolate mixture until glossy and well incorporated. Set aside.
04 - In a chilled bowl, beat cold heavy cream with electric beaters until soft peaks form when lifters are raised.
05 - Using a clean bowl and beaters, whip egg whites until stiff glossy peaks form when beaters are lifted.
06 - Gently fold one-third of the whipped cream into the chocolate mixture to lighten it. Add remaining cream and fold until just combined. Repeat with egg whites, folding in two additions until no white streaks remain.
07 - Divide half the chocolate mousse evenly among 4 serving cups or glasses. Sprinkle a layer of cooled pistachio crunch over each portion. Top with remaining mousse, smoothing surfaces. Refrigerate for at least 2 hours or until set.
08 - Whip cold heavy cream, powdered sugar, and rosewater together until soft peaks form. Refrigerate until ready to serve.
09 - Remove chilled cups from refrigerator. Top each with a generous dollop of rosewater cream. Garnish with additional chopped pistachios and dried rose petals if desired. Serve immediately.