Separate the whites from the yolks and whip the whites until fluffy, gradually adding sugar 1 spoon at a time, until thick and stable.

Add the yolks and whip again, then turn off the mixer, sift in the flour and add the water.

Mix quickly and spread the batter on a parchment-lined baking sheet.

Bake for 12-15 minutes at 170-180 °C. Let the finished sponge cool on the counter - there's no need to roll it, it will cool and come away from the parchment on its own.

Whip the mascarpone for a couple of minutes.

Add the strained sour cream (or 33%+ heavy cream) and powdered sugar and whip for another couple of minutes.

Soak the sponge with the sweet coffee.

Spread the filling over it, leaving one edge bare; set some filling aside for decoration.

Roll it up, wrap in plastic wrap and chill in the fridge for an hour.

Remove, decorate with powdered sugar, cocoa and piped swirls of filling.