Preheat your oven to 350°F (175°C). In a mixing bowl, combine crushed chocolate sandwich cookies with melted unsalted butter until crumbly. Press this mixture firmly into the bottoms of individual serving molds or ramekins to form a solid crust. Bake for 8-10 minutes until slightly set, then remove and let them cool completely before filling.

In a saucepan, combine fresh or frozen blackberries, granulated sugar, water, and fresh lemon juice over medium heat. Cook gently for about 10-15 minutes, stirring occasionally, until the blackberries have softened and released their juices. Once done, strain the mixture through a fine mesh sieve to remove solids, ensuring a smooth puree for your Mini Blackberry Mousse Cakes.

While the blackberry puree cools, sprinkle unflavored gelatin over cold water in a small bowl and let it bloom for 5 minutes. Gently heat the gelatin mixture in the microwave for about 10-15 seconds until fully dissolved. Stir this gelatin into the warm blackberry puree until well combined.

In a heatproof bowl, melt white chocolate chips with ½ cup of heavy cream using a double boiler or in the microwave at 30-second intervals, stirring until smooth. Allow the mixture to cool slightly before combining it with the blackberry mixture, ensuring a creamy and rich flavor to your mousse.

In a separate bowl, whip the remaining heavy cream to soft peaks. Gently fold the whipped cream into the blackberry-white chocolate mixture using a spatula, being careful not to deflate the mousse.

Once everything is folded together, evenly spoon the mousse over the cooled chocolate cookie crusts in your molds. Cover them with plastic wrap and refrigerate the Mini Blackberry Mousse Cakes for at least 4-6 hours, or overnight if possible.

To serve, gently demold the chilled mousse cakes by running a knife around the edges and carefully lifting them out. Garnish with fresh blackberries, edible violets, or pansies for a touch of elegance.