In a microwave safe bowl, melt the butter with the sugar and cocoa powder on HIGH for 30 seconds. Stop, stir, and heat for another 30 seconds.
Stir in the vanilla and salt for about a minute to cool off the butter, then stir in the egg.
Add the flour and stir vigorously for at least 30 seconds (helps the brownies bake up chewy).
Transfer the mixture into the prepared pan, and bake for 24 to 28 minutes. It’s okay for the brownie to stay a little gooey in the center.
Let the brownie cool completely while you make the mousse.
Make the chocolate mousse: Chop the chocolate with a serrated knife. Also make chocolate shavings with a vegetable peeler for the decoration. Set both aside.
Beat the egg with the sugar for about 2 to 3 minutes with a hand mixer, until it’s pale yellow and foamy. Beat in ½ cup of the heavy cream just for a few seconds to combine.
Pour the mixture into a heavy-bottomed saucepan and place over medium heat. Cook, stirring constantly, as the mixture starts to thicken, about 3 to 4 minutes. It will start to coat the back of a spoon when it’s thick enough, but don't let it boil. Thickening the mousse will allow it to hold its shape when the dessert is unmolded.
Remove from heat. Add the chopped chocolate to the pan, and cover. Let the mixture sit undisturbed for 1 minute, then remove the lid and whisk very well to dissolve the chocolate. Pour this mixture into a bowl and chill it for about an hour to firm up.
Beat the remaining ½ cup of cream in a clean bowl until soft peaks are just starting to form.
Remove the chocolate mixture from the refrigerator, and break it up into bite-sized pieces. Begin adding these chocolate mixture pieces to the bowl while beating the cream until it's smooth and fluffy. Add the vanilla extract to the mixture, and beat just to combine.
Assemble the cake: Spoon the mousse on top of the brownie in the springform pan and refrigerate for at least an hour before serving.
When ready to serve, remove the cake from the springform pan by opening the clasp, placing one hand under the pan, and let the rim of the pan fall around your arm.
Top the cake with the shaved chocolate pieces, serve, and enjoy!
Dark chocolate (60 to 72%) and semi-sweet will work as well. Either way, use a good quality chocolate (not chocolate chips).The brownie can be made up to a day ahead.You can just make the mousse and serve it in a bowl or individual ramekins if you'd like. But really, chocolate mousse and brownie cake deserve to be together!