In a large bowl, beat egg yolks until thick and lemon-colored. Gradually beat in 1/4 cup sugar and vanilla. In a small bowl, beat egg whites until soft peaks form. Gradually add remaining sugar, beating until stiff peaks form. Fold into egg yolk mixture. Combine the flour, baking powder and salt; fold into egg mixture.
Line a greased 15x10x1-in. baking pan with waxed paper and grease the paper. Spread batter evenly into pan. Bake at 375° for 12-15 minutes or until top springs back when lightly touched. Cool for 5 minutes. Turn cake onto a kitchen towel dusted with confectioners' sugar. Gently peel off waxed paper. Roll up cake in towel, jelly-roll style, starting with a short side. Cool completely on a wire rack.
For frosting, combine the sugar, cornstarch, salt and water in a saucepan until smooth. Bring to a boil; cook and stir for 2 minutes or until thickened. Remove from the heat. Add the chocolate, butter and vanilla; stir until melted.
While frosting is warm, unroll cake and spread half of frosting over cake to within 1/2 in. of edges. Roll up. Spread remaining frosting over outside of roll. Using tines of a fork, drag along sides of cake to resemble tree bark. Refrigerate for at least 1 hour before slicing.