Grease a 15- x 10- x 1-inch jellyroll pan with vegetable oil, and line with waxed paper.
Grease waxed paper lightly; set aside.
Beat egg yolks in a large bowl, and beat until foamy at high speed of electric mixer; gradually add 1/2 cup sugar, beating until mixture is thick and lemon colored.
Add cocoa, vanilla, and salt; beat at low speed of electric mixer until blended.
Beat egg whites (at room temperature) at high speed of electric mixer until foamy.
Gradually add 1/4 cup sugar, 1 tablespoon at a time, beating until stiff peaks form.
Fold egg whites into chocolate mixture.
Spread batter evenly in prepared pan.
Bake at 375° for 15 minutes.
Sift powdered sugar in a 15- x 10-inch rectangle on a towel.
When cake is done, immediately loosen from sides of pan, and turn out onto sugar.
Peel off waxed paper.
Starting at long side, roll up cake and towel together; cool on a wire rack, seam side down.
Unroll cake, and remove towel.
Spread Mocha Cream Filling over cake, and reroll.
Place on a serving plate, seam side down.
Chill.
Before serving, sift additional powdered sugar over roll.