Preheat oven to 350°F (175°C). Line jelly roll pan with parchment paper.

Beat egg whites with a pinch of salt until soft peaks form. Gradually add half sugar until stiff peaks form.

Whisk egg yolks with remaining sugar until pale. Stir in vanilla extract.

Sift flour and cocoa powder; fold into yolk mixture until smooth.

Fold in egg whites gently in three additions.

Spread batter evenly in pan. Bake 12–15 minutes until springy to touch.

Invert cake onto a powdered sugar-dusted towel, peel off parchment, and roll while warm. Cool completely.

Beat butter, powdered sugar, cocoa powder, and vanilla. Add cream for spreadable consistency.

Unroll cooled cake, spread filling leaving ½-inch border, and roll tightly.

Frost outside with remaining buttercream. Create bark texture with fork. Dust with powdered sugar and decorate.

Chill at least 30 minutes before slicing and serving.