Make the chocolate mousse: Put the chocolate in a food processor fitted with the metal chopping blade. Process for 20 to 30 seconds, until finely ground. In a small saucepan, combine the milk, sugar and salt. Cook over medium heat, stirring with a wooden spoon, until the sugar dissolves and the milk comes to a boil. Remove the pan from the heat. With the motor of the food processor running, pour the hot milk through the feed tube. Process for 10 to 20 seconds, until the chocolate is completely melted. Using a spatula, scrape down the side of the work bowl. Add the oil and vanilla extract; process for 5 to 10 seconds, until the mixture is creamy. Scrape the chocolate mixture into a large bowl and cool for about 5 minutes, until tepid. In a chilled 4-1/2 quart bowl of a heavy-duty electric mixer fitted with the wire whisk attachment, whip the cream until soft mounds barely start to form and the cream is still pourable. Do not overwhip the cream. Using a large rubber spatula, gently fold one-third of the whipped cream into the tepid chocolate mixture to lighten it. Fold in the remaining whipped cream. Do not overfold the mousse or the texture will be grainy. Cover with plastic wrap and chill for 6 to 8 hours or overnight, until firm. Make the cake: Grease three 10-inch round cake pans. Cut three 10-inch circles of waxed paer and place in the bottom of each pan. Grease the pan well and then flour, tapping out the excess. Set aside. Place the oven racks two shelves apart. Preheat the over to 350 degrees F. Sift together the flour, baking soda and salt. Set aside. Place the coarsely chopped chocolate and the water on top of a double boiler and turn heat to medium-high, whisking occasionally until smooth. Remove from the top of the double boiler and cool slightly. Meanwhile, in a 4-1/2 quart bowl of a heavy-duty electric mixer using the paddle attachment, cream together the butter and the sugars. Add the vanilla. Add the eggs one by one, mixing on medium speed until blended. Using a rubber spatula, scrape down the sides of the bowl. With the mixer at its lowest speed, add half of the dry ingredients and the half of the buttermilk. When blended, add the remaining buttermilk and the chocolate mixture. Mix until blended then add the remaining dry ingredients. Mix on medium speed just until blended and the flour lumps disappear. Divide evenly among the three prepared pans and smooth the tops with an offset metal spatula. Bake for 28 to 32 minutes, or until a toothpick comes out clean. Remove the pans from the oven and place on wire cooling racks. Cool for 15 minutes and invert. Peel off the waxed paper and cool completely. Begin to assemble the cake: Place a dab of chocolate mousse and then a layer of the cooled cake on a 10-inch cardboard cake circle or the circular insert of a tart pan. Using a large rubber spatula, place about one-quarter of the chilled chocolate mousse on top of the first cake layer. Using a metal cake spatula, spread the mousse evenly not quite to the sides of the cake. There should be slightly less than one-half inch of mousse on top of the cake. Place the second cake layer on top and repeat the mousse layer. Place the third cake layer on top, then spread the remaining chocolate mousse over the top and sides of the cake, coating smoothly and evenly. Place the cake in the freezer for a minimum of four hours. Make the ganache glaze: Place the coarsely chopped chocolate in a food processor fitted with the metal chopping blade. Process for 20 to 30 seconds, until finely ground. Place in a large bowl. In a medium pan, scald the heavy cream and pour over the chocolate. Using a metal whisk, gently stir until smooth and melted. Continue stirring until the ganache is room temperature or just slightly warmer. Glaze and garnish the cake: Place the Oreos in a food processor fitted with the metal chopping blade. Process for 20 to 30 seconds, until finely ground. Remove the cake from the freezer and place on a wire rack that has been set over aluminum foil or a sheet pan. Slowly pour the ganache over the top of the cake, smoothing with a large metal cake spatula. When the top has been coated, slowly pour the ganache around the sides of the cake so it drips down and coats the sides. Use the spatula to spread the ganache evenly over the sides of the cake. When completely coated with the ganache, scoop the Oreo crumbs with your hand and press them onto the sides of the cake, coating its entirety. Refrigerate until ready to serve.