Make the syrup. Boil the water with the sugar until dissolved. Pour in the alchermes and let it cool down.
Make the sponge. Preheat oven to 250°C.
Whip the egg whites with 75 g of sugar until thick, firm and glossy.
In another bowl whip the egg yolks with 50 g of sugar until light and pale. Fold gently the two masses together.
Sift flour with potato starch and fold it gently into the batter.
Line with parchment paper a baking tray and grease the paper with butter.
Scrape the batter onto the tray and spread evenly: it should be about 1 cm thick.
Bake for about 5 minutes, until slightly golden.
Have ready a dampened kitchen towel and lay it on the table.
As soon as the sponge is ready remove it from the oven and turn it upside down onto the dampened kitchen towel.
Gently peel off the parchment paper and roll the sponge cake, being careful not to break it. Leave it like this for a few minutes.
Prepare the filling by mixing ricotta with chestnut jam.
Unroll the sponge and brush with the Alchermes syrup.
Spread the sponge evenly with the ricotta and chestnut spread then roll again and secure tight with cling film.
Prepare the frosting.
Bring to the boil the cream and pour it over the finely chopped chocolate. Stir until the chocolate has melted, let it cool down and whip with a electric mixer until thick.
Spread the chocolate frosting over the sponge roll, draw a bark-like pattern with a fork and dust with cocoa powder. Leave in the fridge for a few hours before serving.
Slice as thick as you want!