Preheat the oven to 350 degrees F. Butter two 8-inch round cake pans, line the bottoms with parchment paper, butter the parchment and dust with flour, tapping out excess.
Whisk the flour, sugar, cocoa powder, baking soda, baking powder and salt together in a large bowl. Whisk the melted butter, buttermilk, egg, vanilla and 1/2 cup water together in another bowl until smooth. Add the wet ingredients to the dry ingredients, whisking until just combined. Divide the batter between the prepared cake pans, smoothing the tops. Bake, rotating the pans halfway through cooking, until a toothpick inserted in the center of each cake comes out clean, about 25 minutes.
Transfer the cakes to a rack and let cool 15 minutes. Invert the cakes on the rack, discard the parchment paper then flip the cakes right-side-up again. Move the rack with the cakes over a baking sheet and let the cakes cool completely.
Make the raspberry mousse: In a small saucepan, mash the raspberries with the sugar and salt and bring to a simmer over medium heat. Cook, stirring often, until thickened and jammy, 6 to 8 minutes. Remove the pan from the heat, press the raspberry mixture through a sieve into the bowl of a food processor and let cool completely.
Combine the gelatin with 1 tablespoon cold water in a small bowl and let stand for 5 minutes to soften. Heat the gelatin in the microwave until it melts completely, about 10 seconds, stirring to dissolve. Add the gelatin mixture, cream cheese and sour cream to the food processor and process until smooth. Scrape the mixture into a bowl and refrigerate, checking every 5 minutes, until the mousse is just set enough to spread, 5 to 15 minutes. (Do not let the mousse firm up too much.)
Place one cake layer on a cake stand and pile the raspberry mousse in the center, spreading it in a thick layer almost to the edge. Top with the remaining cake, aligning the layers directly on top of each other so the cake does not lean. Refrigerate the assembled cake for 1 hour.
Meanwhile, make the chocolate ganache: Bring the cream to a simmer in a small saucepan then remove from heat. Add the chocolate, shaking the pan gently to submerge it. Allow the pan to stand, off the heat, for 3 minutes to melt the chocolate. Whisk the ganache smooth, then refrigerate until it is just thick enough to spread (like mayonnaise), about 5 minutes. (Do not let the ganache harden too much.)
Stir the ganache again and scoop it on top of the cake, spreading it over the top and sides. Refrigerate the cake until the ganache is set, at least 2 hours or overnight.
When measuring flour, we spoon it into a dry measuring cup and level off the excess. (Scooping directly from the bag compacts the flour, resulting in dry baked goods.)
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