Begin by crushing the chocolate sandwich cookies into fine crumbs, using a food processor or a rolling pin. Melt the butter and mix it with the crumbs until well combined. Press the mixture firmly into the bottom of mini tart pans or silicone molds, ensuring an even layer. Place the molds in the freezer for about 15 minutes, allowing the crust to firm up while you prepare the mousse.

In a medium saucepan over medium heat, combine the fresh blackberries, granulated sugar, and water. Cook for 5-7 minutes, stirring occasionally, until the blackberries soften and release their juices. Remove from heat and let it cool slightly before blending until smooth. Strain the mixture through a fine sieve into a bowl to remove the seeds, creating a luscious blackberry purée.

Sprinkle the gelatin over the fresh lemon juice in a small bowl and let it sit for about 5 minutes. Meanwhile, gently warm the mixture in the microwave for 10-15 seconds or on the stovetop until it becomes liquid. Once it cools to room temperature, blend this gelatin mixture into your prepared blackberry purée, thoroughly combining both elements for a smooth texture.

In a large mixing bowl, whip the heavy cream until you achieve soft peaks. This should take about 2-3 minutes using an electric mixer. Gradually fold the cooled blackberry mixture into the whipped cream, using a spatula. Be gentle to maintain the airy texture of the mousse, ensuring the mixture is smooth and fluffy.

Once your mousse is ready, gently pour it over the chilled crusts in the molds. Use a spatula to smooth the tops, creating an even surface. Cover the molds with plastic wrap and refrigerate for at least 4 hours, but preferably overnight, allowing the mousse to set and develop its divine flavors.

In a small saucepan, heat the heavy cream over medium heat until it just begins to simmer. Remove from heat and add the white chocolate chips, stirring continuously until they melt and form a smooth glaze. Let the glaze cool for a few minutes before using it to ensure it has a perfect consistency for drizzling over the chilled mousse cakes.

Carefully unmold your Mini Blackberry Mousse Cakes by gently pressing the bottoms of the molds or running a knife around the edges. Once they are released, drizzle the glossy white chocolate glaze over each mousse cake. For a decorative touch, garnish with fresh blackberries or edible flowers.