This mousse whips up in 25 minutes and is a light and airy treat for any occasion.
In a small bowl, combine lemon juice and 1/4 cup cold water. Sprinkle gelatin on top and let sit until softened, 2 minutes. Fill a medium bowl with ice water. In a small saucepan, combine gelatin mixture, sugar, and pinch of salt over medium heat and stir until gelatin and sugar dissolve, about 4 minutes. Pour gelatin mixture back into small bowl and set in ice bath. Stir until mixture reaches room temperature, about 1 minute.
In a large bowl, whisk cream until soft peaks form. Pour cooled gelatin mixture into whipped cream and whisk until soft peaks return. Divide evenly among four 8-ounce dishes or cups and place in freezer until mousse springs back lightly when pressed, 10 minutes. Serve topped with blackberries.
To make this dessert ahead of time, reduce the amount of gelatin to 1 1/2 teaspoons and let the mousse set in the refrigerator for 1 1/2 hours.