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, chilled bowl. Whip with 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 the mixture becomes pale yellow and thickened, approximately 3-4 minutes. Incorporate orange blossom water and lemon zest, whisking until fully blended.
04 - Process the cooled date mixture in a food processor or blender until completely smooth, scraping down sides as needed. Fold the date paste into the egg yolk mixture until uniformly combined.
05 - Add one-third of the whipped cream to the date-yolk mixture, folding gently with a spatula to lighten the base. Fold in remaining cream in two additions until smooth and airy, taking care not to deflate the mixture.
06 - Using a completely clean bowl and whisk attachment, beat egg whites until stiff, glossy peaks form. Gently fold the whites into the mousse base in two additions, using a lifting motion to preserve maximum air volume.
07 - Divide the mousse evenly among individual serving glasses or bowls, filling about 3/4 full. Smooth tops with the back of a spoon. Refrigerate for a minimum of 2 hours until completely set and firm.
08 - Just before serving, sprinkle each portion with chopped pistachios and finely grated orange zest. Arrange additional sliced dates on top if desired. Serve chilled.