Nonstick vegetable oil spray
2 cups unbleached all purpose flour
1 3/4 cups sugar
3/4 cup unsweetened cocoa powder
2 teaspoons baking soda
1/4 teaspoon salt
1 cup water
3/4 cup buttermilk
3/4 cup vegetable oil
3 large eggs
Chocolate Ganache and Raspberry Topping:
18 ounces bittersweet chocolate (do not exceed 61% cacao), chopped
2 1/4 cups heavy whipping cream
6 tablespoons seedless raspberry jam, stirred to loosen, divided
2 6-ounce containers fresh raspberries
Special equipment: 2 9-inch-diameter cake pans with 2-inch-high sides
Position racks in top and bottom third of oven; preheat to 350°F. Coat two 9-inch-diameter cake pans with 2-inch-high sides with nonstick spray. Line bottoms with parchment paper rounds; spray rounds (which will help let you get the cake out of the pans).
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).
Bake cakes until tester inserted into center comes out clean, about 30 minutes.
For chocolate ganache and raspberry topping:
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.
Invert 1 cake layer onto cardboard round or bottom of 9-inch-diameter tart pan with removable bottom. Peel off parchment paper.
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. Freeze to set ganache, about 30 minutes.
Arrange raspberries in concentric circles atop cake. Sift powdered sugar lightly over raspberries and serve.