Preheat oven to 400°F. Spray 9 x 13 in. silicone jelly roll pan with vegetable pan spray; line with parchment paper and spray again. Place on cookie sheet.
In small bowl, sift together flour and cocoa powder.
In large bowl, whip egg yolks and 1/3 cup sugar with an electric mixer on high speed until thick and pale yellow, about 2 minutes, scraping down sides and bottom of bowl as necessary.
In large bowl, combine egg whites, 1/4 cup sugar and salt. Whip on high speed with electric mixer until foamy; add cream of tartar and vanilla. Continue whipping until the whites are glossy and form stiff peaks, about 3 minutes. Carefully fold 1/3 beaten egg whites into yolk mixture until just combined. Gently fold in remaining egg whites and flour mixture until there are no streaks visible. Spread into prepared pan.
Bake 10-12 minutes or until toothpick inserted in center comes out clean. Remove cake from oven; remove silicone pan to cooling grid and let cool 5 minutes. Dust top of cake with cocoa powder. Use pan to help roll cake from short end. Transfer rolled cake to cooling grid and let cool completely.
When ready to assemble, gently unroll cake, remove parchment paper, fill and re-roll cake. Ice outside of cake with chocolate icing.
*Tip: if removing the parchment paper and rolling the cake in the pan is difficult, turn cake out from pan onto cocoa powder-dusted parchment paper to remove parchment paper backing.