Preheat the oven to 350 degrees F. (Or whatever temperature is listed on the back of the brownie mix.) Combine the brownie mix, oil, water, and 2 eggs in a mixing bowl. Whisk well. Then pour the batter into a greased 9x13-inch baking dish. Bake for 25 minutes, or as listed on packaging. Cool the brownies completely before moving on. You can place the pan in the refrigerator to speed up the cooling if needed.
Meanwhile, melt the chocolate in a double-boiler or in the microwave. Stir it until smooth. Then allow it to cool to room temperature.
Place the butter, sugar, vanilla, and salt in the bowl of an electric stand mixer. Beat on high until light and fluffy, at least 3-5 minutes. Then drizzle in the melted chocolate and beat until the mixture is even.
Scrape the bowl with a spatula and turn on high again. Add the remaining 4 eggs, one at a time, allowing the mixture to beat a solid 3 minutes after adding each egg, 12 minutes total. Stop and scrape the bowl as needed.
Spread the chocolate mousse over the top of the cooled brownies. Using a veggie peeler, shave chocolate curls over the top if desired.
Chill for 2 hours, or until ready to cut and serve.
These chocolate mousse brownies are ultra creamy and fudgy... Meaning hard to cut! 1) Either cut with them a knife, wiping the blade between cuts. 2) Or for you perfectionists, line the pan with parchment paper before baking the brownies. Once the brownies are chilled and set, lift the whole sheet of brownies out of the pan by the edges of the paper. Then use dental floss held tight between both hands to cut straight lines down through the brownies.