Separate the whites from the yolks and whip the whites in a grease-free bowl until they clump, about 5 minutes.

Add the sugar gradually, whip until thick, add the yolks and whip again.

Turn off the mixer, sift in the flour and cocoa, add the water and essence, and fold in quickly and gently.

Pour the batter onto a baking sheet lined with white parchment and bake for 18-20 minutes at 160-170 °C. Let the finished sponge cool completely - there's no need to roll it.

Drain the rich 21% sour cream on cheesecloth for a couple of hours, until the whey separates.

Add the powdered sugar to the drained mixture and whip by hand with a whisk.

Spread the filling over the sponge, stopping short of the edge.

Roll it up tightly, wrap in plastic wrap and refrigerate for a couple of hours to soak through.

Dust with cocoa or powdered sugar, slice and serve.