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Grease and flour three 9" round cake pans.
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Trace three 9-inch circles on parchment paper; cut out. Draw 6-inch circle, centered, on each cutout; draw 3-inch circle, centered, on each cutout.
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Line bottom of each pan with 1 paper circle, pencil side down.
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In large heatproof bowl set over hot (not boiling) water, melt chopped chocolate. Stir in cocoa powder. Let cool to room temperature.
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In separate bowl, combine flour, baking powder, baking soda and salt. Set aside.
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In large bowl, beat butter with sugar until light and fluffy. Beat in eggs,one at a time, beating well after each addition, add vanilla.
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Beat in flour mixture alternately with buttermilk, making 3 additions of dry ingredients and 2 of buttermilk.
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Stir 3 1/2 cups of the batter into melted chocolate.
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Spoon white and chocolate batters into separate large piping bags(or spoon into 2 large resealable plastic bags and cut 1-inch hole in 1 corner of each).
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Pipe chocolate batter to fill outer ring of each of 2 of the pans.
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Pipe white batter over middle ring, then chocolate batter over inner ring.
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In third pan, pipe white batter over outer ring, chocolate over middle ring and white over inner ring.
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Bake in centre of 350°F oven until cake tester inserted in centre comes out clean, 20-25 minutes.
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Let cool in pans on rack for 10 minutes. Turn out onto rack; let cool completely.
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For the filling, place chopped chocolate in bowl.
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In saucepan, heat whipping cream over medium heat just until steaming. Pour over chocolate, whisking just until melted.
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Refrigerate until thickened but still spreadable, about 30 minutes.
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Whisk just until color lightens.
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Using serrated knife, trim tops of cake layers to make level, if necessary.
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Starting with 1 of the cake layers with chocolate outer ring, spread with 1/3 cup of the ganache.
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Top with cake layer with white outer ring; spread with another 1/3 cup of the ganache.
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Top with remaining cake layer.
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Cover top and side with remaining ganache.
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Refrigerate before serving.
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Some people may find it easier to use one of "Checkerboard Cake Pan Sets" available online or in local stores.
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http://www.amazon.com/s/102-7283235-2715300?ie=UTF8&keywords=Checkerboard%20Cake%20Pan&search-type=ss&index=kitchen.