Begin by crushing chocolate sandwich cookies into fine crumbs using a food processor. Melt unsalted butter in a microwave-safe bowl, then combine it with the cookie crumbs until well blended. Firmly press this mixture into the bottom of silicone or ring molds to create a sturdy crust. Place the molds in the freezer for 15 minutes to set the crust properly.

In a saucepan over medium heat, combine fresh or frozen blackberries, sugar, and water. Cook until the berries are broken down and bubbly, about 5-7 minutes. Blend the mixture into a smooth purée and strain it through a fine mesh sieve. Bloom gelatin in lemon juice, warm it slightly, then stir into the blackberry purée. Fold in vanilla extract and whipped cream until light and smooth.

Once the chocolate crust has set, pour the blackberry mousse over it in the molds, distributing evenly. Gently tap the molds on the counter to release any air bubbles. Cover with plastic wrap and refrigerate for at least 4 hours, or until the mousse is firm.

To create a luscious glaze, heat heavy cream in a small saucepan until it simmers, then pour over white chocolate chips in a bowl. Stir until fully melted and smooth. Let the glaze cool slightly before pouring.

After the mousse cakes have set, carefully unmold them onto serving plates. Pour the slightly cooled glaze over each cake, ensuring an even coating. Top each cake with a fresh blackberry and an edible flower just before serving.