To prepare the brownie: Preheat oven to 300° F. Line a 10-inch baking pan with 3-inch sides with parchment paper and spray the sides with cooking spray.
In a large bowl, combine chocolate chips and sugar. Add melted butter and mix with an electric mixer for 2-3 minutes. Add the eggs one at a time and stir after each addition until well combined.
Sift the cake flour and cocoa and add to the mixture. Scrape the sides of the bowl and beaters to combine. Mix until it is a smooth paste. Pour into the baking pan and bake for 45 minutes, or until the center springs back when pressed.
Let the brownie cool completely, then run a knife around the edges to loosen and remove from pan. Turn upside down into a clean 10-inch baking pan with 3-inch sides lined with parchment or a similar size springform pan.
To prepare the cheesecake: Preheat oven to 285° F.
With an electric mixer, beat cream cheese. Add sugar and continue to mix. Scrape the sides of the bowl and beaters to combine and eliminate lumps.
Add the eggs one at a time and stir after each addition. Add vanilla extract and mix 1 minute. Pour on top of the cooked brownie in cake pan. Bake the cheesecake/brownie on a rimmed sheet or baking pan in a 1/2 inch water bath for 45 minutes. The top should feel firm to the touch. Let cool completely.
To prepare the chocolate mousse: In a double boiler or bowl over simmering water, melt the chocolate. In a separate bowl, beat the egg yolks and sugar with an electric mixer until light and fluffy.
Slowly alternate the addition of cream and melted chocolate to the egg yolks. Beat until soft peaks form. Pour the chocolate mousse on top of the cheesecake (you might have some extra mousse). Smooth the surface to avoid air pockets.
Refrigerate until the mousse sets, about 3 hours. If not using a springform pan, run a knife around the edges to loosen. Heat the bottom and the sides of the pan on the stovetop for a few seconds. Turn it upside down on a piece of plastic and cardboard. Shake the pan, and the cake should slowly come out. Flip it back on a piece of cardboard or serving plate and remove the plastic.Makes 8 to 12 Source: