Preheat the oven to 350 F. Line a 9-inch cake pan with parchment and spray well with nonstick cooking spray. Whisk together the flour, cocoa powder, salt, and baking powder in a small bowl. Set aside for now.
Cube the butter and place it in a medium saucepan over medium-low heat. Stir occasionally as the butter melts. Add the sugar and stir it into the melted butter until combined.
Remove the pan from the heat, and let it cool to lukewarm. Once it has cooled, add the eggs, one at a time, whisking well between each addition. The mixture will start out grainy, but as you add the eggs, it will become shiny and smooth. Whisk the vanilla in. Finally, add the dry ingredients, and stir them in with a spatula.
Scrape the brownie batter into the prepared pan, and bake it for 22-24 minutes, until a toothpick comes out with just a few moist crumbs attached. Cool the brownie layer completely before proceeding.
To Make the Mousse Layers:
Prepare your pan. You need a 9-inch pan with sides at least 3 inches tall, and the ability to remove the bottom. Either a cake pan with a removable bottom or a springform pan will work. Line the inner wall with a 4-inch high acetate cake collar or strip of aluminum foil or waxed paper. Make sure the acetate or waxed paper is long enough so that it overlaps itself, and if you are using foil/waxed paper, make sure you cut the strip tall enough to extend at least an inch over the pan’s sides, since the cake is about 4 inches tall.
Place a cake cardboard in the bottom of the pan, then carefully place the baked brownie on top of the cardboard. Prepare the chocolate mousse layer: combine the chocolate, 3/4 cup of cream, and salt in a microwave-safe bowl. Microwave in 30-second increments, stirring every 30 seconds, until the chocolate has melted and the mixture is smooth. Pour it into a large bowl, and let it cool to room temperature, stirring occasionally.
While you wait for the chocolate mixture to cool, prepare the gelatin. Whisk together the gelatin and the cold water in a small bowl, and set it aside to let the gelatin absorb the water. When the chocolate is at room temperature, microwave the bowl of gelatin for 15 seconds, until it is melted. Whisk the melted gelatin and chocolate together.
Whip the remaining 1 cup heavy cream to firm peaks. Fold half of the whipped cream into the chocolate, and once it’s incorporated, gently fold in the remaining whipped cream. Scrape the mousse on top of the brownie, and spread it into an even layer. Refrigerate the cake for 25 minutes, until the mouse has started to set and is firm enough that a second layer can be added.
For the raspberry mousse, make a quick puree by blending 1/2 cup fresh raspberries in a blender or food processor. Pour them through a fine mesh strainer to remove the seeds, and measure out 1/4 cup raspberry puree. (If you have extra puree, save it for another use.)
Combine the white chocolate, raspberry puree, 1/2 cup of heavy cream, and salt in a microwave-safe bowl. Continue to make this layer the same way you made the chocolate layer, by melting and cooling the chocolate, whisking in melted gelatin, and folding in whipped cream. Add a few drops of pink food coloring to boost the pink color, if desired. At the very end, stir in the remaining 1 cup of fresh raspberries and pour the mousse over the chocolate mousse layer. Spread it into an even layer, and refrigerate again.
Finally, prepare the vanilla bean layer. Combine the white chocolate, 1/2 cup heavy cream, honey, and salt in a microwave-safe bowl. Continue to make this layer the same way you made the previous two layers. At the very end, stir in the vanilla bean paste and pour the mousse over the raspberry mousse layer. Refrigerate to set the mousse layers very well, at least 4 hours or overnight.
To finish the cake, push the bottom out from the pan, or unhinge the sides, and unwrap the acetate strip. Transfer the cake to your serving platter. Make a quick chocolate ganache by putting the chopped chocolate in a small bowl and heating the cream to a simmer. Pour the hot cream over the chopped chocolate and whisk gently until the chocolate melts and you have a smooth, shiny mixture.
Pour the ganache over the top of the cake, and use a spatula to nudge it right to the edge and over in evenly spaced drizzles. Don’t worry if the top is not smooth—it will be covered up! Add the fresh raspberries and chocolate curls on top of the cake. Refrigerate to set the ganache.
For the cleanest slices, cut the cake when it is well chilled. Use a large sharp knife, and wash it frequently between cuts. For the best taste and texture, allow the cake slices to sit at room temperature for 10-15 minutes before serving.
Store Chocolate Raspberry Mousse Cake, well-wrapped, in the refrigerator for up to a week.