Prepare a 10-inch diameter round tart pan, preferably with a removable bottom, and lightly grease with butter.
Finely grind the gluten-free Oreo-style cookies in a food processor until no lumps remain. Add the soft butter to the crushed cookies and the melted, cooled dark chocolate. Mix thoroughly until all ingredients are well combined. Press the mixture firmly into the prepared tart pan, evenly covering the bottom and sides. Place in the refrigerator to set.
For the milk chocolate layer, melt the milk chocolate in a double boiler. Remove from heat. Remove 1-2 tablespoons melted chocolate for decoration. To the remaining melted chocolate, add a tablespoon of heavy cream and 1 teaspoon powdered gelatin. Mix thoroughly until all ingredients are well incorporated. Allow the mixture to cool, stirring occasionally to prevent it from setting. Add the cold cream cheese to the cooled chocolate mixture and blend until you achieve a fluffy and smooth consistency.
In a large, very cold metal bowl, whip ¾ cup (minus 1-2 tablespoons) cold heavy cream until it forms stiff peaks.
Gently fold the whipped cream into the chocolate mixture using a large spoon or spatula. Spread the chocolate mousse evenly over the chilled crust in the tart pan. Return to the refrigerator.
For the white chocolate layer, repeat the same process as the milk chocolate layer, also reserving 1-2 tablespoons melted chocolate for decoration.
Carefully spread the white chocolate mousse over the milk chocolate layer in the tart pan. Using the reserved milk and white melted chocolates, create small dots on the top layer and then create decorative patterns by dragging a toothpick or a small fork through the chocolate.
Cover the pie and refrigerate for at least 5 hours, or until the mousse is set. This pie can be prepared up to 2 days in advance.