01 - Combine chopped dates and 2 tbsp 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 allow to cool completely to room temperature.
02 - Pour cold heavy cream into a large, chilled bowl. Using an electric mixer or whisk, beat until soft peaks form. Transfer to refrigerator to maintain volume until needed.
03 - Whisk egg yolks and granulated sugar in a separate bowl until mixture becomes pale yellow and noticeably thickened, approximately 3-4 minutes. Add orange blossom water and lemon zest, stirring until fully incorporated.
04 - Process the cooled date mixture in a food processor or blender until completely smooth with no visible chunks. Gently fold the date puree into the egg yolk mixture until uniform in color and texture.
05 - Add one-third of the whipped cream to the date-yolk mixture, folding gently with a spatula until partially incorporated. Add remaining cream in two more additions, folding carefully until smooth and airy, being careful not to deflate the mixture.
06 - In a thoroughly clean, grease-free bowl, whisk egg whites until stiff, glossy peaks form. Gently fold the beaten egg whites into the mousse base in two additions, using a light hand to preserve maximum air volume.
07 - Spoon or pipe the finished mousse into individual serving glasses or dessert bowls. Place in refrigerator and chill for at least 2 hours, or until completely set and firm to the touch.
08 - Just before serving, sprinkle chopped pistachios over the surface of each mousse. Top with freshly grated orange zest and additional sliced dates if desired. Serve chilled.