Preheat the oven to 350°F (175°C).
Line a 15-in. x 10-in. x 1-inch baking pan with greased parchment paper.
Lay a clean kitchen towel flat on a work surface. Dust with powdered sugar and set aside.
Place egg whites in a small bowl and set aside.
Place egg yolks in a large bowl and beat with a stand mixer until fluffy and lighter in colour, about 3 minutes. Gradually add half the sugar and beat until thick and even lighter in colour, about 5 minutes. Gradually add flour, cocoa and salt, beating until well blended. Set this batter aside.
Beat egg whites on medium speed until foamy. Add cream of tartar and beat until soft peaks form. Gradually sprinkle in remaining sugar, a tablespoon at a time, beating on high until stiff peaks form.
Transfer a quarter of the egg white mixture into the batter, stirring to just combine. Gently fold in remaining egg whites until no light streaks remain.
Spread batter evenly (using an offset spatula) in the prepared pan. Bake for about 15 minutes or until cake springs back when lightly pressed in the center (do not over bake or cake will crack when you roll it).
Remove from the heat and let cool on a rack for 3 minutes. Run a small knife along all four sides to loosen. Invert onto a prepared kitchen towel. Peel off the parchment paper and loosely roll the cake up in a towel, beginning with a short side. Set aside to cool.