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1
Preheat the oven to 425u00b0 F. Grease a very flat 12- by 17-inch baking sheet with butter or nonstick spray. Line the baking sheet with parchment (cut slits in the paper at the corners to prevent bunching of excess paper) and grease the parchment.
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2
In a small pot, bring the water to a boil. Pour the boiling water over the cocoa powder in a small, heat-safe bowl. Bring to room temperature (or refrigerate for 5 to 10 minutes). Stir in the vanilla.
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3
In a medium bowl, combine the 2 whole eggs and 3 egg yolks with 2 1/2 ounces of the sugar. In another medium bowl, place the 3 egg whites with the cream of tartar, reserving the remaining sugar.
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4
Using an electric mixer, whip the whole eggs and yolks to ribbon stage (until the mixture is pale yellow and thick), 3 to 5 minutes.
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5
Gently fold the cocoa mixture into the whole egg mixture. Sprinkle the flour into the whole egg mixture as you fold it gently, until fully incorporated.
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6
With clean whips, whip the egg whites with an electric mixer until frothy, about 1 minute. Gradually add the remaining 2 1/2 ounces sugar and continue to whip to stiff peaks, 4 to 5 minutes.
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7
Temper the whole egg mixture with 20% of the egg whites, mixing somewhat vigorously to combine. Gently fold in the remaining whites in two additions, mixing until just combined.
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8
Gently pour the batter into the prepared pan and use an offset spatula to smooth the batter into an even layer in the pan.
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9
Bake until the cake springs back slightly when touched, 10 to 12 minutes. Do not overbake.
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10
When the cake comes out of the oven, sift an even layer of cocoa powder all over the surface. Use a paring knife to loosen the cake and parchment from the edges of the pan, then roll up the cake tightly, using the parchment to assist you if the cake is too hot to touch easily. For a traditional roulade, roll from the longer side. For a vertical roulade, roll from the shorter side. Cool completely in the parchment paper cocoon.
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11
In the bowl of an electric mixer, whip the cream, sugar, and vanilla to medium peaks.
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12
To make a horizontal (traditional) the roulade, unroll the roulade and spread an even layer of whipped cream in the center. Re-roll the roulade tightly. Chill until ready to serve.
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13
To make a vertical roulade, unroll the roulade, and use a ruler to help divide it into three even strips (about 3 3/4 inches long). Cut the strips with a pastry wheel. I used a pastry bag to pipe the whipped cream onto each strip, but you can also just dollop and spread.
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14
Starting with one strip, roll it up tightly into a roulade starting on the shorter side. Place that roulade at the end of the next strip, and continue rolling. Place the roll at the end of the final strip, and continue rolling. Turn the entire roulade on its flat side to create a cake with vertical layers.
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15
Frost the outside of the cake with whipped cream and chill until ready to serve.
