1/3 to 1/2 cup fresh lemon juice, strained
2 1/4 teaspoons unflavored powdered gelatin (from 1 packet)
1/2 cup sugar
1 cup cold heavy cream
1 cup fresh blackberries (6 ounces)
1. In a small bowl, combine lemon juice and 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.
2. 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.