Preheat oven to 350F and line a jelly-roll pan with parchment.

Beat the egg yolks with sugar until pale, then fold in the cocoa and flour.

Whip the egg whites to stiff peaks and gently fold them in.

Spread into the pan and bake 10-12 minutes, then roll up warm in a towel to cool.

Whip the cream with powdered sugar and vanilla, unroll the cake, and spread the filling.

Re-roll the cake, frost with ganache, drag a fork for bark texture, and chill 1 hour before serving.