Make The Brownie Layer: whisk both sugars, vegetable oil, butter, and cocoa powder in a small pot over low heat. Remove from heat once the mixture is smooth.
Pour the batter into the prepared pan. Bake for 20-25 minutes or until the center is just set. Cool to room temperature then transfer to a springform pan.
Make the Chocolate Mousse: whip the heavy cream medium high until soft peaks form. Transfer to the fridge.STEP 5Soak the gelatin in cold water and set aside.
Melt the chocolate and water over a double boiler. Whisk until smooth then set aside.STEP 7Use a stand mixer fitted to whip the egg yolks until thickened and pale yellow.
While the yolks are whipping, combine the sugar and water in small pot. Cook until it reaches 235°F. Slowly stream the the sugar syrup into the whipping egg yolks.
Evenly spread the chocolate mousse on top of the cooled brownie in the prepared springform pan. Transfer to the fridge to set for at least 6 hours or overnight.