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1
Position racks in top and bottom third of oven; preheat to 350u00b0F. Coat two 9-inch-diameter cake pans with 2-inch-high sides with nonstick spray. Line bottoms with parchment paper rounds; spray rounds. Sift flour, sugar, cocoa powder, baking soda, and salt into large bowl; whisk to blend and form well in center. Whisk 1 cup water, buttermilk, oil, and eggs in medium bowl to blend. Pour wet ingredients into well in dry ingredients; whisk just to blend. Divide cake batter between prepared pans (about 3 cups each).
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2
Bake cakes until tester inserted into center comes out clean, about 30 minutes. (If cakes form domes, place kitchen towel atop hot cakes, then press gently with palm of hand to level.) Cool completely in pans on cooling racks. DO AHEAD:
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3
Place chopped chocolate in medium bowl. Bring cream just to boil in heavy medium saucepan. Pour over chocolate. Let stand 1 minute, then stir until ganache is melted and smooth. Transfer 1 1/4 cups ganache to small bowl. Cover and refrigerate until ganache is thick enough to spread, stirring occasionally, about 1 hour. Let remaining ganache stand at room temperature to cool until barely lukewarm.
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4
Place rack inside rimmed baking sheet. Carefully run knife around pan edges to release cakes. Invert 1 cake layer onto cardboard round or bottom of 9-inch-diameter tart pan with removable bottom. Peel off parchment paper. Place cake layer on round on prepared rack. Spread 3 tablespoons jam over top. Spoon dollops of chilled ganache over, then spread evenly. Invert second cake layer onto another cardboard round or tart pan bottom. Peel off parchment paper. Carefully slide cake off round and onto frosted cake layer on rack. Spread remaining 3 tablespoons raspberry jam over top of second cake layer. Pour half of barely lukewarm ganache over cake, spreading over sides to cover. Freeze until ganache sets, about 30 minutes. Pour remaining ganache over cake, allowing to drip down sides and spreading over sides if needed for even coverage and to smooth edges. Freeze to set ganache, about 30 minutes. DO AHEAD:
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5
Arrange raspberries in concentric circles atop cake. Sift powdered sugar lightly over raspberries and serve.
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6
A two-step process ensures a picture-perfect dessert. First, a thin layer of ganache is spread over the cake and chilled briefly to set (this is called a crumb coat). Another layer of ganache is then poured over the cake, which gives the treat a smooth finish.
