Preheat your oven to 350°F (175°C).
Prepare the crust by mixing graham cracker crumbs, melted butter, and sugar in a bowl.
Press the crust mixture into the bottoms of small cake pans.
Bake the crust for 10 minutes, then let it cool.
In a small bowl, combine the blackberries, sugar, and lemon juice, then mash gently.
In a separate bowl, whip the heavy cream until soft peaks form.
Gently fold the blackberry mixture into the whipped cream until just combined.
Spoon the blackberry mousse over the cooled crusts in the cake pans, smoothing the tops.
Chill the mousse cakes in the refrigerator for at least 4 hours, or until set.
Once set, carefully remove the cakes from the molds and serve.