Preheat oven to 350° F. Line a straight-sided 9x13-inch baking pan with parchment paper (being sure to leave some of the parchment paper to overhang the sides of the pan) and lightly spray the sides with baker’s spray. This will make it easier to lift the brownies out of the pan to better slice and serve them. Set aside.

To a large mixing bowl, add the melted unsalted butter and granulated sugar. Whisk until the sugar has dissolved in the warm melted butter.

Add the vanilla extract and eggs, one at a time, whisking until each egg is fully incorporated before adding the next egg. Whisk until smooth and all the ingredients are fully incorporated.

To the butter mixture, add the cocoa powder, all-purpose flour, and salt. Whisk gently, being careful not to overmix, until all the dry ingredients are evenly mixed into the wet ingredients and the brownie batter is smooth and thick.

Spread the brownie batter into the prepared baking pan and bake for 30-35 minutes or until a toothpick inserted in the center comes out clean or with a couple of moist crumbs stuck to the toothpick.

Allow the brownies to cool completely on the counter before adding the chocolate mousse layer.

To a small bowl add the cold water and unflavored gelatin. Gently stir to combine then set aside to allow the gelatin to absorb the water completely.

To a 1-cup glass, microwave-safe, liquid measure cup, (or a small microwave-safe bowl) add ½ cup of the heavy cream. Microwave on high for 45 seconds to 1 minute to heat the heavy cream until steamy. You do not want to boil the heavy cream.

Using a fork, or small whisk, stir the gelatin into the warm heavy cream until it is fully dissolved.

To a medium bowl, add the finely chopped semi-sweet chocolate, then pour the warm heavy cream over the chopped chocolate. Be sure that you cover all the chocolate with the warm cream, and allow it to sit for 1-2 minutes or until the chocolate has melted. Stir until the chocolate is smooth and all the heavy cream has been incorporated. You may need to heat the chocolate and heavy cream for another 30-40 seconds, in the microwave to get the chocolate warm enough to stir to a smooth consistency.

Set aside to allow the chocolate to cool completely to room temperature. This is very important for the next step, as you do not want to add warm chocolate to the whipped cream for the mousse mixture.

In a large mixing bowl add the remaining 2 cups of heavy whipping cream and beat on high speed, using a handheld mixer starting on low speed and working up to high speed as the cream starts to thicken, and beat the heavy cream until soft peaks start to form. This should take 2 - 2 ½ minutes.

Add the powdered sugar and vanilla extract to the whipped cream then continue to beat the whipped cream for another 2 - 2 ½ minutes or until stiff peaks form.

Add ⅓ of the room temperature melted chocolate to the whipped cream and very gently fold it into the whipped cream using a rubber spatula.

Add another ⅓ of the melted chocolate to the whipped cream and very gently fold it in.

Finally, add the remaining ⅓ of the melted chocolate to the whipped cream, and fold until no white streaks remain and the chocolate mousse mixture is light and fluffy.

Using a small offset spatula, spread the chocolate mousse mixture evenly over the top of the cooled brownie pan.

Cover the baking pan with plastic wrap, being sure not to disturb the chocolate mousse layer, and refrigerate the chocolate mousse brownies for a minimum of 6-8 hours or up to overnight. You want to allow the chocolate mousse to set-up to a firm consistency so that you get nice clean cuts when sliced and served.

Once the mousse layer has fully set, lightly dust the top of the brownies with 1 tablespoon of additional unsweetened cocoa powder to garnish (do not do this until ready to serve), then run a sharp knife around the edges of the baking pan, to loosen the sides of the brownies, before gently lifting the chocolate mousse brownies out of the pan using the overhanging parchment paper. Using a large sharp knife, cut the brownies into 12 (3x3 inch) servings.