Melt the butter gently over a water bath until fully liquid. Add the melted butter to the Oreo crumbs and mix until every crumb is evenly coated.

Pour the mixture into the bottom of a 6-inch springform pan or cake ring placed on a parchment-lined plate. Using the bottom of a glass or a measuring cup, press the crumbs down firmly and evenly to create a compact base. Refrigerate for at least 10 minutes to set while you make the mousse.

Place the gelatin sheets in a small bowl of cold water to soften for about 5 minutes.

Combine the chopped dark chocolate and milk in a heatproof bowl. Set the bowl over a pot of gently simmering water (double boiler), ensuring the bottom of the bowl doesn't touch the water. Stir frequently until the chocolate is completely melted and the mixture is smooth.

Remove the bowl from the heat. Squeeze out any excess water from the softened gelatin sheets and add them to the warm chocolate mixture. Stir vigorously until the gelatin is fully dissolved. Set this aside and let it cool to room temperature. It should feel just slightly warm to the touch.

In a separate large bowl, combine the cold heavy cream and sugar. Using an electric hand mixer or a whisk, whip the cream until it reaches soft to medium peaks—it should be thick, hold its shape, and have a creamy, spreadable consistency like thick yogurt. Be careful not to overwhip.

Pour the cooled (but still liquid) chocolate mixture into the whipped cream. Whip the two together until you have a uniformly chocolate-colored, smooth mousse with no streaks of white cream.

Remove the prepared cookie base from the fridge. Pour the chocolate mousse over the base, spreading it evenly with a spatula. Gently tap the pan on the counter a few times to release any air bubbles.

Carefully transfer the cake to the refrigerator. Chill for at least 4 hours, or ideally overnight, until the mousse is completely firm and set.

Combine the dark chocolate and heavy cream in a bowl and melt over a water bath until smooth and glossy. Let the glaze cool to about body temperature — too hot and it will melt the mousse underneath.

Pour the glaze gently over the set mousse cake, allowing it to flow naturally over the surface. Tilt the pan slightly if needed to help it spread evenly.

Return the cake to the refrigerator for another 20 minutes, just until the glaze is set.