Prep oven to 350°F. Line a 10x15" jelly roll pan with parchment and lightly grease it.

Make sponge: Beat egg yolks with both sugars until pale and thick (about 3 minutes). Add vanilla. In a separate bowl, sift together flour, cocoa, baking powder, and salt.

In another bowl, beat egg whites to stiff peaks. Gently fold flour mixture into yolk mixture, then fold in egg whites in 3 additions.

Bake: Spread batter evenly in pan. Bake for 10-12 minutes, until springy.

Roll: While warm, invert onto a tea towel dusted with powdered sugar. Peel off parchment and roll cake with towel inside. Let cool completely.

Make filling: Beat cream, sugar, and vanilla until soft peaks form.

Unroll, fill, re-roll: Gently unroll cooled cake, spread filling, and re-roll without towel. Chill 30 minutes.

Make ganache: Heat cream until steaming, pour over chocolate and butter. Let sit 2 min, then stir smooth.

Decorate: Spread ganache over cake, drag fork through to make bark texture. Chill to set.