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1
Position rack in center of oven. Preheat oven to 350u00b0F. Bake 30 minutes for layers, 38 to 40 minutes for tube cake. Prepare the pan(s) as directed.
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2
In a medium bowl, whisk together the flour, baking soda, salt and cocoa. Set aside.
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3
In the large bowl of an electric mixer, cream together the butter and sugar for 3 to 4 minutes, until very well blended. Scrape down the bowl and beater. Beat in the vanilla and eggs and scrape down the bowl and beater again.
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4
With the mixer on the lowest speed, alternately add the flour mixture and the buttermilk. Once the ingredients are blended together, increase the speed and whip for about 30 seconds (no longer, because at high altitudes you don't want to incorporate excess air).
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5
Divide the batter between the two pans or scrape it all into the tube pan. Bake 30 minutes for layers, 38 to 40 minutes for tube cake (or for the time indicated for your altitude in the chart), or until the cake top feels springy to the touch and a cake tester inserted in the center comes out clean. Cool in the pan(s) on a wire rack for 10 to 15 minutes.
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6
Run a knife between the layers and the pan sides to release them, or run the tip of a knife around the pan sides and the top of the tube to loosen the cake. Top each layer, or the tube pan, with a foil-covered cardboard cake disk or flat plate, invert, and give a sharp downward shake to release the cake. Remove the pan and peel off the parchment. Cool completely.
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7
In a food processor or the large bowl of an electric mixer, preferably with the paddle attachment, process or beat the butter until soft. Add 2 cups of the sifted sugar and beat until smooth. Scrape down the bowl and blade or beaters. Add the remaining 4 cups sugar, plus cocoa, 5 tablespoons coffee, and vanilla, and process or beat until completely smooth and creamy. Add more sugar or coffee if necessary to bring the icing to spreading consistency. (The icing can be made a day in advance and refrigerated, covered; bring to room temperature and beat until smooth before using.)
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8
Fill the layers and frost with the buttercream, or dust the tube cake lightly with cocoa or confectioners' sugar (or frost as desired).
