Preheat your oven to 350°F (175°C). In a mixing bowl, combine crushed chocolate sandwich cookies with melted unsalted butter. Press this mixture into the bottom of individual molds to create a solid base. Bake for 8-10 minutes until slightly crisp, then let cool completely.

In a saucepan, combine blackberries, granulated sugar, water, and lemon juice over medium heat. Cook for 5-7 minutes until the berries break down. Strain through a fine sieve to achieve a smooth blackberry puree.

Sprinkle unflavored gelatin over 2 tablespoons of cold water in a small bowl. Allow to sit undisturbed for about 5 minutes until it blooms. Heat the blackberry puree until just warm, then mix in the bloomed gelatin until fully dissolved.

In a heatproof bowl over simmering water, combine white chocolate chips and some heavy cream. Stir until melted and smooth. Let it cool to room temperature.

In a mixing bowl, combine the cooled blackberry puree, melted white chocolate, and vanilla extract. Gently fold in whipped heavy cream until the mixture is smooth and airy.

Pour the blackberry mousse over the cooled crusts, smoothing the tops. Refrigerate for at least 4-6 hours, preferably overnight, until set.

Once set, carefully demold the cakes, place on plates, and garnish with blackberries and optional edible flowers. Serve chilled.