Preheat oven to 350°F (175°C) and line a 15×10-inch (38×25 cm) pan with parchment.
Beat egg yolks and sugar until light; add vanilla, then fold in sifted flour, cocoa, and salt.
Whisk egg whites to soft peaks; gently fold into batter. Spread in pan and bake for 10–12 minutes.
Invert cake onto a sugared towel, peel parchment, roll up to cool. Whip cream with powdered sugar for filling.
Unroll cake, spread with whipped cream, re-roll, chill. Melt chocolate and butter for ganache, cool slightly, then spread over cake. Dust with powdered sugar before serving.
Notes
Ensure the cake is completely cooled before applying ganache for best results.