Preheat oven to 350°F (180°C). Line a 12 x 17 inch baking pan with parchment paper. Let some parchment paper hang off the sides, to make it easier to lift the cake out once baked. Set aside.
Using a fine mesh sieve, sift four, cocoa powder, baking powder, and salt into a medium sized bowl. Set aside.
Use a hand mixer or stand mixer, beat egg whites and ¼ cup of sugar until medium peaks form. Transfer egg whites into another bowl. Set aside.
Using the same bowl you had the egg whites in (no need to clean it), add egg yolks and remaining sugar. Beat until egg yolks are lightened in colour, about 1 minute beating on high.
Add in melted butter, vanilla, and buttermilk. Mix to combine.
Add ⅓ egg whites to the egg yolk mixture. Use a rubber spatula to gently mix the egg whites in.
Add in ½ of the dry ingredients, mix to combine. Add in another ⅓ egg whites and gently mix to combine. Then add the remaining ½ of the dry ingredients, followed by the remaining ⅓ of egg whites. Mix gently to combine.
Spread batter evenly into prepared pan. Use an offset spatula to get batter to all corners.
Bake for 12-14 minutes, or until the cake springs back when lightly touched with your finger.
While cake is baking, lay out a kitchen towel and sprinkle with cocoa powder.
Once the cake comes out of the oven, run a knife down the sides to loosen it, and immediately turn out onto the kitchen towel. Carefully peel off the parchment that was at the bottom of the cake.
Start at the narrow end and tightly roll the cake with the towel. Allow the cake to cool to room temperature, about 2 hours.