While the brownie is cooling:
Prepare your pan. You need a 9-inch pan with sides at least 3 inches tall, and the ability to remove the bottom. Either a cake pan with a removable bottom or a springform pan will work. Butter or spray the pan lightly with nonstick spray, then line it with cling wrap that extends up and over the sides. Line the Inner wall with an acetate cake collar or strip of waxed paper. Make sure the acetate or waxed paper is long enough so that it overlaps itself, and if you are using waxed paper, make sure you cut the strip tall enough to extend at least as high as the pan’s sides.
Carefully place the baked brownie in the bottom of the pan. Prepared the chocolate mousse layer; combine the chocolate, 170 g (3/4 cup) of cream and salt in a microwave safe bowl. Microwave is 30 second increments, stirring every 30 seconds, until the chocolate has melted and the mixture is smooth. Pour it into a large bowl and let it cool to room temperature, stirring occasionally.
While you wait for the chocolate mixture to cool, prepare the gelatin. Whisk together the gelatin and the cold water in a small bowl and set it aside to let the gelatin absorb water. When the chocolate is at room temperature, microwave the bowl of gelatin for 10-15 seconds, until it is melted. Whisk the melted gelatin and chocolate together.
Whip the remaining heavy cream to firm peaks. Fold half of the whipped cream into the chocolate, and once it’s incorporated, gently fold in the remaining whipped cream. Scrape the cake for 25 minutes, until the mousse has started to set and is firm enough that a second layer can be added.