Prepare the Blackberry Puree: In a small saucepan, combine the blackberries, granulated sugar, and lemon juice. Cook over medium heat for 5–7 minutes, stirring occasionally, until the berries are soft and juicy.

Strain the Mixture: Pour the cooked berries through a fine-mesh strainer into a clean bowl. Press with a spoon to extract as much juice and pulp as possible. Discard the seeds. Let the puree cool completely at room temperature.

Whip the Cream: In a large mixing bowl, whip the cold heavy cream with powdered sugar and vanilla extract until stiff peaks form. Do not over-whip.

Fold the Puree into Cream: Gently fold the cooled blackberry puree into the whipped cream using a spatula. Continue folding until evenly combined and fluffy.

Assemble the Mousse: Spoon or pipe the mousse into dessert glasses or bowls. Smooth the tops if needed.

Chill the Dessert: Refrigerate for at least 2 hours to allow the mousse to set properly.

Garnish and Serve: Before serving, top with fresh blackberries, a swirl of whipped cream, and mint leaves for decoration.