In a medium saucepan, combine the bittersweet morsels, milk, and chunks of salted butter. Heat the mixture over low heat, stirring constantly until smooth and melted. Once you have a glossy chocolate mixture, remove it from the heat.

Stir in the instant espresso powder and miniature marshmallows to your chocolate mixture while it's still warm. Keep stirring until the marshmallows dissolve, giving the chocolate an extra creamy texture and a hint of coffee flavor.

In a large mixing bowl, pour in 1 cup of heavy whipping cream and the vanilla extract. Using an electric mixer, whisk until soft peaks form. This means the cream will hold its shape but still be soft enough to fold into the chocolate later on.

When your chocolate mixture is cool (but not hardened), gently fold in the whipped cream. Be careful not to deflate the whipped cream too much; you want that light, airy texture.

Once thoroughly mixed, pour the chocolate mousse into the pre-made Oreo cookie crust. Smooth the top with a spatula. Then, cover it lightly and chill it in the refrigerator for at least 4 hours, or preferably overnight. This allows the mousse to set beautifully.

Before serving, whip the remaining heavy cream and add a touch of sugar if desired. Spread or pipe this whipped cream on top of the chilled mousse pie. Finish with a sprinkle of orange zest and chocolate curls for that extra pizazz!