Grease and lightly flour a 15x10x1-inch jelly-roll pan; set aside. Stir together flour and salt; set side. Beat egg yolks and sherry with an electric mixer on high speed about 5 minutes or until thick and lemon-colored. Gradually add 1/2 cup of the granulated sugar, beating until sugar is almost dissolved.
Thoroughly wash beaters. In a very large bowl beat egg whites and cream of tartar on medium to high speed until soft peaks form (tips curl). Gradually add the remaining granulated sugar, 2 tablespoons at a time, beating on medium to high speed until stiff peaks form (tips stand straight). Fold 1 cup of the egg-white mixture into egg-yolk mixture. Fold egg-yolk mixture into remaining egg-white mixture. Fold in flour mixture; spread in prepared pan.
Bake in a 375 degree F oven for 12 to 15 minutes or until top springs back when lightly touched. Immediately loosen edges of cake from pan. Turn cake out onto a towel sprinkled with powdered sugar. Roll up warm cake and towel, jelly-roll style, starting from a short side. Cool on a rack.
Gently unroll cake. Spread Coffee-Cream Filling on cake to within 1 inch of the edges. Roll up cake without towel, jelly-roll style, starting from 1 of the short sides. Cut a 1-1/2-inch slice from 1 end of cake. Frost cake with Rich Chocolate Frosting. Place the slice on side of log to form a branch. Frost branch. Using the tines of a fork, score the cake lengthwise to resemble tree bark. Makes 10 servings.
Rich Chocolate Frosting
Heat and stir chocolate and butter in a saucepan until chocolate melts. Remove from heat; stir in 1-1/2 cups sifted powdered sugar, vanilla, and 1/4 cup milk. Add 1-1/2 cups additional sifted powdered sugar and enough milk to make of spreading consistency (about 1 to 2 tablespoons). Makes about 1- 1/2 cups.
Beat whipping cream, powdered sugar, and instant coffee crystals in a mixing bowl until soft peaks form. Makes about 2 cups.