01 - Combine chopped dates and water in a small saucepan. Cook over low heat for 3-4 minutes, stirring constantly until dates soften into a jam-like consistency. Remove from heat and let cool completely to room temperature.
02 - In a chilled large bowl, whip cold heavy cream using an electric mixer or whisk until soft peaks form. Refrigerate immediately to maintain structure.
03 - Whisk egg yolks and granulated sugar in a separate bowl until mixture becomes pale yellow and thickened, approximately 3-4 minutes. Incorporate orange blossom water and lemon zest until fully blended.
04 - Process cooled date mixture in a food processor or blender until completely smooth, forming a uniform paste. Fold the date paste thoroughly into the egg yolk mixture until no streaks remain.
05 - Gently fold one-third of the whipped cream into the date-yolk base to lighten the mixture. Add remaining cream in two additions, folding with a spatula using circular motions to maintain airiness until smooth and uniform.
06 - Using a clean bowl and whisk, beat egg whites until stiff, glossy peaks form. Carefully fold whipped egg whites into the mousse base in two additions, using minimal folding motions to preserve maximum volume.
07 - Distribute mousse evenly among individual serving glasses or bowls. Refrigerate for a minimum of 2 hours until completely set and firm to the touch.
08 - Top chilled mousse with chopped pistachios, finely grated orange zest, and additional sliced dates if desired. Serve immediately while cold.