Combine the chocolate and butter in a saucepan or in a microwave-safe bowl. Melt over low heat (on the stovetop) or on 50% power in the microwave at 1-minute increments, stirring in between.
Cool the mixture to lukewarm, then whisk in the sugar until smooth.
Add the eggs one at a time, blending well after each addition. Mix in the vanilla and salt. Add the flour and mix until just combined.
Pour the batter into the prepared pan and smooth. Bake for 30-35 minutes, until the cake just rises in center (a tester inserted into center will not come out clean) and springs back lightly to the touch.
Cool the cake in the pan completely (the center may fall which is normal).
Make the Chocolate Mousse:
Melt the chocolate - over low heat (on the stovetop) or on 50% power in the microwave at 30-second increments, stirring in between.
Pour the melted chocolate into the bowl of a stand mixer. Add the powdered sugar, cream cheese and vanilla and mix until completely smooth, scraping down the sides and bottom 1-2 times.
With the mixer running on low, slowly pour in the heavy cream. Scrap down the sides and bottom of the bowl, then increase the speed of the mixture to medium-high and whip until light and fluffy, about 3-4 minutes.
Spread the mousse on top of the cooled chocolate cake. Cover and refrigerate for at least 6 hours or up to 1 day.
Make the Whipped Cream:
Using an electric mixer, beat the cream and powdered sugar in a medium bowl until soft peaks form. Spread the whipped cream over the cake. Dust with grated chocolate, if desired. Chill for up to an hour or serve immediately.
Heavy Cream: Be sure to use “heavy cream” because it contains a milk fat percentage of 35% (or higher). Many brands of heavy whipping cream only have 30-35% milk fat, whereas heavy cream has 35% or higher. This mousse needs the extra milk fat to be sure it turns out thick and creamy. I prefer Darigold heavy cream (40% milk fat). Not all brands or cartons label the milk fat percentage, so to be safe, look for a brand that has 35% (or higher) written on the carton. Chocolate Chips: A high-quality chocolate chip works best here. I like Guittard Semi-Sweet best!Chocolate Shavings: I like to make simple chocolate shavings using a bar of chocolate and a vegetable peeler to decorate this cake.