Step 1: Prepare the Blackberry Puree

1. In a small saucepan, combine the blackberries, granulated sugar, and lemon juice.
2. Cook over medium heat for about 5-7 minutes, stirring occasionally. The blackberries should soften and release their juices.
3. Once the berries have broken down, remove from heat.
4. Strain the mixture through a fine-mesh sieve, using a spoon to press out as much juice as possible. This step removes the seeds, ensuring a smooth mousse.
5. Let the blackberry puree cool to room temperature.

Step 2: Whip the Cream

1. In a large mixing bowl, add the cold heavy whipping cream, powdered sugar, and vanilla extract.
2. Using an electric mixer, whip the mixture on medium-high speed until stiff peaks form. This should take about 3-4 minutes.
3. Be careful not to overwhip, as this can cause the cream to turn into butter.

Step 3: Combine the Mousse

1. Gently fold the cooled blackberry puree into the whipped cream using a spatula.
2. Fold in slow, circular motions to maintain the airy texture.
3. Continue folding until the color is evenly distributed and no streaks remain.

Step 4: Assemble the Mousse

1. Spoon or pipe the mousse into serving cups or dessert bowls.
2. Smooth the tops with the back of a spoon for a clean presentation.

Step 5: Chill and Serve

1. Refrigerate the mousse for at least 2 hours to allow it to set. This step is crucial for achieving the perfect texture.
2. Before serving, garnish with fresh blackberries, a dollop of whipped cream, and a few mint leaves for an elegant touch.