01 - Beat the softened cream cheese with a hand mixer in a medium bowl until smooth and fluffy, approximately 2 minutes.
02 - Add granulated sugar, lemon zest, lemon juice, and vanilla extract to the cream cheese mixture. Beat until fully incorporated and creamy, about 1 minute.
03 - In a separate chilled bowl, whip the cold heavy cream with powdered sugar using a hand mixer until stiff peaks form, approximately 3-4 minutes.
04 - Gently fold the whipped cream into the lemon cheesecake mixture using a spatula. Add the sour cream and fold until just incorporated, being careful not to deflate the mixture.
05 - Spoon the mousse into individual serving glasses or bowls, filling each about three-quarters full to allow room for garnishes.
06 - Cover the glasses or bowls with plastic wrap and refrigerate for at least 2 hours, or until the mousse is firm and set.
07 - Remove from refrigeration and top with additional lemon zest, fresh berries, and crushed graham crackers or shortbread cookies if desired. Serve chilled.