Bûche de Noël is a traditional French Christmas cake shaped and decorated to look like a tree log. It's filled and frosted with silky chocolate buttercream. Crunchy meringue mushrooms are the perfect contrast. Chopped pistachios scattered around resemble moss growing on the (chocolate) log.
Grease bottom and sides of a 15- x 10-inch jellyroll pan; line with wax paper, and grease and flour wax paper. Set aside.
Beat egg yolks in a large mixing bowl at high speed with an electric mixer 5 minutes or until thick and pale. Gradually add 1/3 cup sugar, beating well. Add water and vanilla. Fold in ground almonds. Gradually fold in cake flour and cocoa.
Beat egg whites at high speed until foamy. Add cream of tartar and salt; beat until soft peaks form. Add remaining 1/3 cup sugar, 1 tablespoon at a time, beating until stiff peaks form. Gently fold into egg yolk mixture.
Spread batter evenly into prepared pan. Bake at 375° for 10 minutes or until top springs back when touched.
Sift powdered sugar in a 15- x 10-inch rectangle on a cloth towel. When cake is done, immediately loosen from sides of pan, and turn out onto sugared towel. Peel off wax paper. Starting at narrow end, roll up cake and towel together; cool completely on a wire rack, seam side down.
Unroll cake, and remove towel. Spread cake with half of Rich Chocolate Buttercream; carefully reroll. Cover and chill. Cut a 1-inch-thick diagonal slice from 1 end of cake roll.
Place cake roll on a serving plate, seam side down; position cut piece against side of cake roll to resemble a knot. Spread remaining Rich Chocolate Buttercream over cake.
Score frosting with the tines of a fork or a cake comb to resemble tree bark. Garnish with pistachios and, if desired, Meringue Mushrooms. Store cake (but not mushrooms) in refrigerator.