Preheat the oven to 350 degrees F. Butter two 8-inch square 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.
Let the cakes cool in the pans for 15 minutes, then turn them out onto a cooling rack set over a baking sheet. Discard the parchment paper and flip the cakes right side up again. Let the cakes cool completely.
Make the caramel creme: Heat 1/2 cup sugar in a small nonstick skillet over medium-high heat. Cook, stirring with a rubber spatula, until it turns dark amber in color, 4 to 5 minutes. Remove the pan from the heat and pour in the heavy cream (mixture will bubble up). When the bubbles subside, add the remaining 1/4 cup sugar and the salt and stir until both dissolve in the caramel. Scrape the caramel 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 caramel creme filling into a bowl and refrigerate, checking every 5 minutes, until it is just thick enough to spread, 5 to 15 minutes. (Do not let the filling firm up too much.) Before layering the cake, give the mixture a good whisking to aerate it slightly. This will make for a lighter texture.
Place one cake on a cake stand and pile the caramel filling 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 the 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 until smooth, then refrigerate until it is just thick enough to spread (like mayonnaise), about 15 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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