In a saucepan, combine fresh blackberries, granulated sugar, and lemon juice over medium heat. Cook for about 5–7 minutes, stirring gently, until the blackberries soften and release their juices. Once softened, pour the mixture into a blender and pulse until smooth. Strain the purée through a fine mesh sieve to remove any seeds, then let it cool completely before proceeding to the next step.

In a large mixing bowl, ensure your heavy cream is well-chilled, then use an electric mixer to whip it on medium speed. Whip the cream until soft peaks form, which should take about 2-3 minutes. Gradually add in the vanilla extract and powdered sugar, continuing to whip until stiff peaks form—this should take another 1-2 minutes.

Once the blackberry purée has cooled, gently fold it into the whipped cream using a spatula. Start by adding a small amount of whipped cream to the purée, mixing gently to lighten it up. Then gradually fold in the remaining whipped cream, being careful not to deflate the mixture.

Spoon the beautifully blended mousse into individual serving glasses or bowls. Cover these with plastic wrap and refrigerate for at least 2 hours, or until set. When ready to serve, garnish each portion with extra blackberries and fresh mint leaves.