Begin by crushing your chocolate sandwich cookies in a food processor until they resemble fine crumbs. Melt unsalted butter in a microwave-safe bowl, then combine it with the cookie crumbs until fully coated. Press this mixture into silicone or ring molds to form a sturdy base. Place the molds in the freezer for about 15 minutes to firm up the crust before adding the mousse.

In a saucepan over medium heat, combine the fresh or frozen blackberries, sugar, and a splash of water. Cook until the blackberries break down and release their juices, stirring occasionally, for around 10 minutes. Once softened, transfer the mixture to a blender and purée until smooth. Strain the purée through a fine sieve to remove any seeds for a silky texture.

While your blackberry purée cools, bloom the gelatin by soaking it in lemon juice for approximately 5 minutes. Once bloomed, gently warm the mixture in a small saucepan just until the gelatin dissolves completely—be sure not to boil. Stir the dissolved gelatin into the warm blackberry purée until fully incorporated, allowing it to cool slightly before proceeding.

In a separate mixing bowl, whip heavy cream using an electric mixer on medium speed until soft peaks form, about 3-5 minutes. Once whipped, carefully fold the cream into the cooled blackberry mixture along with the vanilla extract. Use gentle motions to maintain the airy texture of the cream, ensuring the mousse is light and fluffy.

Once your mousse mixture is ready, remove the crusts from the freezer. Pour the light and creamy blackberry mousse over the chilled chocolate cookie crusts in the molds, filling them just to the top. Smooth the surface with a spatula and then return the molds to the refrigerator. Allow the mousse to set for at least 4 hours, or overnight for best results.

When the mousse is set, prepare the white chocolate glaze. In a small saucepan, heat additional heavy cream over low heat until it’s warm but not boiling. Pour this cream over white chocolate chips in a separate bowl and stir until melted and smooth. Let the glaze cool slightly until it thickens but is still pourable, about 10-15 minutes.

Once the glaze has cooled adequately, remove the mousse cakes from the refrigerator. Gently unmold each cake and place them on a serving platter. Pour the silky glaze over the top of each mousse cake, allowing it to drip down the sides for an inviting look. Garnish with fresh blackberries and edible flowers for a stunning final touch, and serve chilled.