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 jammy consistency. Remove from heat and allow to cool completely to room temperature.
02 - Pour cold heavy cream into a large mixing bowl. Using an electric mixer or whisk, whip until soft peaks form. Transfer to refrigerator and keep chilled until needed.
03 - Whisk egg yolks and granulated sugar in a separate bowl until mixture becomes pale yellow and thickened, approximately 2-3 minutes. Stir in orange blossom water and lemon zest until fully incorporated.
04 - Transfer cooled date mixture to a food processor or blender. Process until smooth paste forms, scraping sides as needed. Fold pureed dates into egg yolk mixture until uniformly combined.
05 - Gently fold chilled whipped cream into date-yolk base in two separate additions. Use a spatula with lifting motion to maintain airiness, folding just until no white streaks remain.
06 - Using a clean bowl and beaters, whisk egg whites until stiff glossy peaks form. Carefully fold beaten whites into mousse base using gentle folding motion to preserve maximum volume and aeration.
07 - Divide mousse evenly among individual serving glasses or bowls, filling each about three-quarters full. Smooth tops with back of spoon for even presentation.
08 - Refrigerate for minimum 2 hours or until mousse is completely set and holds shape when touched. Cover loosely with plastic wrap if chilling longer than 4 hours.
09 - Just before serving, sprinkle chopped pistachios over top of each portion. Add fresh orange zest and additional sliced dates as desired. Serve chilled.