01 - Combine chopped dates and water in a small saucepan. Cook over low heat for 3-4 minutes, stirring constantly until soft and jammy. Remove from heat and let cool completely to room temperature.
02 - In a large bowl, whip the cold heavy cream using an electric mixer or whisk until soft peaks form. Refrigerate immediately to maintain structure.
03 - Whisk egg yolks with granulated sugar in a separate bowl until pale and thickened, approximately 2-3 minutes. Stir in orange blossom water and lemon zest until fully incorporated.
04 - Transfer cooled dates to a food processor or blender. Process until completely smooth, scraping down sides as needed. Fold the date paste into the egg yolk mixture until uniform.
05 - Gently fold one-third of the whipped cream into the date-yolk base to lighten. Add remaining whipped cream in two portions, folding carefully to maintain airiness until smooth and no streaks remain.
06 - Using a clean bowl and whisk, beat egg whites until stiff, glossy peaks form. Gently fold into the mousse base in two additions, being careful not to deflate the mixture.
07 - Spoon or pipe the finished mousse into individual serving glasses or bowls, filling each about three-quarters full. Smooth tops with an offset spatula if desired.
08 - Refrigerate for at least 2 hours, or until completely set and firm to the touch. Cover loosely with plastic wrap if chilling longer than 4 hours.
09 - Just before serving, sprinkle chopped pistachios and finely grated orange zest over the tops. Add sliced dates for extra sweetness and visual appeal if desired.