Preheat oven to 350°F. Line a 10x15-inch jelly roll pan with parchment paper.
Beat egg yolks and 1/4 cup sugar until thick and pale. Add vanilla extract.
In a separate bowl, sift together flour, cocoa powder, baking powder, and salt.
In another bowl, beat egg whites with cream of tartar until frothy. Gradually add 1/4 cup sugar and beat to stiff peaks.
Gently fold the yolk mixture into the egg whites, then fold in the dry ingredients.
Spread the batter evenly in the prepared pan. Bake for 12–14 minutes.
Invert the cake onto a kitchen towel dusted with powdered sugar. Peel off parchment paper and roll the cake in the towel. Let cool completely.
Whip the cream with powdered sugar and vanilla until stiff peaks form.
Unroll the cake, spread the whipped cream evenly, and roll it back up.
Dust with powdered sugar before serving. Chill until ready to slice.