Preheat the oven to 350 degrees F (or 320 degrees for convection). Line a 9-by-13-inch baking sheet with parchment.
For the base: Melt the chocolate chips in a double boiler set over medium heat. In a separate saucepot melt the butter over medium heat.
While the chocolate and butter are melting, separate the eggs. Place the yolks in a medium bowl and the whites in the bowl of a stand mixer.
Whisk the yolks together with 1/2 cup of the sugar, making sure they are fully combined and the sugar completely dissolved.
Add the remaining 1/2 cup sugar to the whites. Using the whip attachment, beat the whites to medium-stiff peaks.
While the whites whip, combine the melted chocolate, melted butter and the cornstarch in a large bowl. Whisk in the egg yolk mixture, then stir in a scoop of the whipped egg whites to temper. Using a rubber spatula, fold in the remaining whites. Transfer to the prepared baking sheet and spread out in an even layer.
Bake for 10 to 15 minutes. Remove directly to the refrigerator and let cool completely (preferably overnight).
Remove the parchment and cut the base into rounds using a cutter (Freeze excess for next time, or enjoy as brownies.)
For the mousse: Bring the heavy cream to a boil in a medium saucepot over medium-high heat.
Place the chocolate chips in a large bowl and pour over the hot cream. Let stand for 1 minute to melt the chocolate. Stirring from the middle of the bowl outwards, stir together the chocolate and cream until shiny and smooth. Let cool slightly, 3 to 5 minutes.
Whip the heavy whipping cream to stiff peaks. Stir one scoop of the whipped cream into the mousse to temper. Fold in the remaining whipped cream.
Spoon the mousse into silicone molds and top with the cake rounds. Freeze until solid. When completely frozen, unmold and set on a cooling rack over a baking sheet for covering with ganache.
For the ganache coating: Bring the heavy cream to a boil in a medium saucepot over medium heat. Place the chocolate chips in a medium bowl and pour over the hot cream. Let sit for one minute to melt the chocolate Stirring from the middle of the bowl and outwards, stir together the chocolate and cream until shiny and smooth.
Ladle spoonfuls of the ganache over top of the frozen mousse to cover. Garnish with gold leaf if using.