Preheat the oven to 350°F and line a 17x12 inch jelly roll pan with parchment paper, ensuring it sticks up at least an inch above the sides.
Whisk together the flour, cocoa powder, baking powder, and salt in a medium bowl.
In a large bowl, whisk the egg yolks and sugar until well combined. Add sour cream, melted butter, and vanilla, then whisk again.
Gently fold the dry ingredients into the wet ingredients until well combined.
In a separate bowl, whip egg whites on high until stiff peaks form.
Fold one-third of the whipped egg whites into the batter to loosen it, then gently fold in the remaining egg whites.
Spread the batter evenly into the pan and bake for 10-12 minutes, until the cake springs back and a toothpick comes out clean.
After baking, lift the cake out of the pan using the parchment paper, roll it up while still warm, and let it cool completely.