Let’s assemble this decadent Brownie Bottom Chocolate Mousse Cake step-by-step. First, prepare the brownie base. Preheat your oven to 350°F (177°C). Line the bottom of a 9-inch springform pan with a circle of parchment paper. It’s also helpful to line the sides with parchment strips or an acetate cake collar later, before adding the mousse, for easier release.

In a medium-sized bowl, combine the contents of the brownie mix package, the melted butter, the eggs, and the 2 tablespoons of water. Stir with a spatula or wooden spoon until the batter is just combined and you no longer see dry streaks. Be careful not to overmix.

Pour the brownie batter into the prepared springform pan and spread it into an even layer using your spatula. Bake for 20-24 minutes, or until a toothpick inserted into the center comes out clean (or with slightly moist crumbs, depending on how fudgy you like your base). Let the brownie cool completely in the pan on a wire rack. This is crucial; adding mousse to a warm brownie will cause melting.

Now, prepare the components for the chocolate mousse. Ensure your cream cheese is fully softened to room temperature for a smooth result. Start the first gelatin mixture: in a very small dish, place 1 tablespoon of cold water. Sprinkle 1 teaspoon of gelatin evenly over the water. Let it sit for 5-10 minutes until it hydrates and becomes solid or jelly-like (this is blooming).

In a separate small microwave-safe bowl, combine the ½ cup mini chocolate chips and 1 tablespoon of heavy whipping cream. Microwave at 50% power in 30-second increments, stirring after each, until the chocolate is melted and smooth. Once melted, microwave the bloomed gelatin mixture for 8-10 seconds, just until it turns back into a liquid (don’t boil it). Immediately pour the liquid gelatin into the melted chocolate mixture and stir quickly and thoroughly to combine. Set this chocolate-gelatin mixture aside to cool slightly but remain fluid.

Prepare the second gelatin mixture just like the first: sprinkle 1 teaspoon of gelatin over 1 tablespoon of cold water in a small dish and let it bloom. While it blooms, place a medium-sized metal mixing bowl and your mixer’s whisk attachment (if possible) in the freezer for 5-10 minutes to chill them thoroughly.

Pour 1 cup of cold heavy whipping cream into the chilled bowl. Beat on medium-high speed until the cream just starts to thicken. Microwave the second bloomed gelatin mixture for 8-10 seconds until liquid. With the mixer running on medium-low speed, slowly pour the liquid gelatin into the thickening cream. Increase the speed to medium-high. As the cream continues to thicken, gradually add the 3 tablespoons of powdered sugar. Continue beating on high speed until stiff peaks form (the peaks hold their shape and don’t flop over). Be careful not to overbeat into butter. Set this stabilized whipped cream aside (ideally in the fridge).

In a large bowl (or the bowl of your stand mixer with the paddle attachment), beat the softened cream cheese on medium-high speed for 2-3 full minutes, scraping the bowl occasionally, until it is completely smooth with absolutely no lumps. Add the ½ cup of granulated sugar and beat until well combined. Add the ¼ cup of cocoa powder and beat again until combined, scraping the bowl as needed.

With the mixer on low speed, slowly add the remaining 2 tablespoons of heavy whipping cream and the teaspoon of vanilla extract. Beat until just combined. Add the slightly cooled melted chocolate-gelatin mixture to the cream cheese mixture. Mix on medium speed until thoroughly combined and uniform in color.

Now, retrieve the stabilized whipped cream from the fridge. Add about one-third of the whipped cream to the chocolate-cream cheese mixture and gently fold it in using a large spatula to lighten the base. Add the remaining whipped cream and continue to fold gently but thoroughly, scraping from the bottom, until the mixture is evenly combined and no white streaks remain. Be careful not to deflate the whipped cream too much – maintain that airy mousse texture.

Ensure the brownie base is completely cool in the springform pan. If you didn’t line the sides earlier, you might want to do so now with parchment strips or an acetate collar for cleaner removal later. Pour the finished chocolate mousse mixture evenly over the cooled brownie base. Smooth the top with your spatula.

Refrigerate the entire cake for at least 4 hours, or preferably overnight, until the mousse is completely firm and set. Do not rush this step!

Just before serving, prepare the topping. Chill another medium metal bowl and whisk attachment. Pour the ¾ cup of cold heavy whipping cream into the chilled bowl. Beat on medium-high speed until it starts to thicken. Gradually add the 3 tablespoons of powdered sugar and continue beating on high speed until stiff peaks form.

Carefully remove the sides of the springform pan (and the parchment/collar, if used). Transfer the whipped cream to a piping bag fitted with a large open star tip (optional). Pipe a decorative border around the top edge of the cake. Gently warm the hot fudge sauce so it’s drizzly but not hot. Drizzle it over the top of the cake. Sprinkle with the remaining ¼ cup of mini chocolate chips. Slice with a sharp knife (wiping clean between cuts can help) and serve immediately