01 - Combine chopped dates and water in a small saucepan. Cook over low heat for 3-4 minutes, stirring constantly until dates break down into a soft, jammy consistency. Remove from heat and allow to 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. Place in refrigerator to maintain stability while proceeding with next steps.
03 - Whisk egg yolks and granulated sugar in a separate bowl until mixture becomes pale yellow and noticeably thickened. Incorporate orange blossom water and lemon zest, whisking until fully combined.
04 - Transfer cooled date mixture to a food processor or blender. Process until completely smooth, scraping down sides as needed. Gently fold the date paste into the egg yolk mixture until uniform in color and texture.
05 - Remove whipped cream from refrigerator. Gently fold one-third of the cream into the date-yolk base to lighten the mixture. Fold in remaining cream in two additions, using a spatula with gentle lifting motions to maintain airiness.
06 - In a thoroughly clean bowl, whisk egg whites until stiff, glossy peaks form. Carefully fold the whipped egg whites into the mousse base using gentle folding motions, being careful not to deflate the mixture.
07 - Spoon or pipe the mousse into individual serving glasses or bowls, dividing evenly among portions. Refrigerate for minimum 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 finely grated orange zest and additional sliced dates if desired. Serve chilled.