Chocolate-Orange Bûche de Noël

Preheat oven to 350 °F. Butter 15 x 10 x 1-inch jelly-roll pan. Line bottom with waxed paper, allowing 1-inch overhang on short sides. Butter paper. Lightly dust paper and pan with flour; tap out excess. Using electric mixer, beat egg whites in large bowl until foamy. Add cream of tartar and beat until soft peaks form. Gradually add 6 tablespoons sugar, beating until stiff peaks form. Set aside. Beat egg yolks, 1/3 cup cocoa powder and remaining 6 tablespoons sugar in another large bowl until thick, about 2 minutes. Stir 1/4 of whites into yolk mixture to lighten. Fold in remaining whites. Gently spread batter in prepared pan. Bake until cake springs back when pressed in center, about 15 minutes. Transfer pan to rack. Cool completely. Sift remaining 1 tablespoon cocoa powder over cake. Line baking sheet with foil. Invert cake onto sheet. Peel off paper.