In a saucepan, combine fresh blackberries, granulated sugar, and lemon juice. Cook over medium heat for about 5-7 minutes until softened. Blend until smooth, strain to remove seeds, and cool completely.

In a mixing bowl, whip the chilled heavy cream until soft peaks form. Add vanilla extract and powdered sugar, then beat until sturdy peaks form.

Gently fold the cooled blackberry purée into the whipped cream mixture, retaining airiness.

Spoon into serving vessels and refrigerate for at least 2 hours to set. Garnish with additional blackberries and mint leaves before serving.