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1
Make cake layer: Preheat oven to 350u00b0F Oil a 15- by 10- by 1-inch shallow baking pan and line bottom lengthwise with a large piece of wax or parchment paper, letting paper hang over ends by 2 inches.
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2
Melt chocolate with water in a small heavy saucepan over very low heat, stirring. Cool to lukewarm.
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3
Beat yolks, 1/3 cup sugar, and salt in a large bowl with an electric mixer until thick and pale, about 5 minutes in a standing mixer or about 8 minutes with a hand-held mixer. Fold in melted chocolate until blended. Beat whites with cleaned beaters until they just hold soft peaks. Gradually add remaining 1/3 cup sugar and beat until whites just hold stiff peaks. Fold one third of whites into melted-chocolate mixture to lighten, then fold in remaining whites gently but thoroughly.
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4
Spread batter evenly in baking pan and bake in middle of oven until puffed and top is dry to the touch, 15 to 18 minutes. Transfer pan to a rack. Cover top with 2 layers of damp paper towels and let stand 5 minutes, then remove towels and cool completely. Loosen edges with a sharp knife.
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5
Sift cocoa powder over top of cake layer and overlap 2 layers of wax paper lengthwise over cake. Place a baking sheet over paper and invert cake onto it, gently peeling off wax paper lining. (Don't worry if cake layer breaks; it will hold together when rolled.).
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6
Make filling: Beat cream with confectioners sugar and Grand Marnier with cleaned beaters until it just holds stiff peaks. Fold in zest.
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7
Fill and roll cake: Spread filling evenly over cake. Put a long platter next to a long side of cake. Using wax paper as an aid, roll up cake jelly roll-style, beginning with a long side. Carefully transfer, seam side down, to platter, using wax paper to help slide cake. (Cake will crack but will still hold together.).
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8
Dust cake generously with cocoa powder and confectioners sugar.
