Make the syrup. Boil the water with the sugar until dissolved. Pour in the alchermes and let it cool down.
Make the chocolate filling. Bring the milk to a boil with the vanilla seeds and set it aside.
Whisk the eggs with the sugar and cornstarch, then pour the milk little by little and stir to mix thoroughly. Put the custard back on the stove on low heat and stir with a whisk until it gets firm.
Remove the custard from the heat and add the chopped chocolate, stirring until it is completely melted, then let cool.
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.
Unroll the sponge and brush with the Alchermes syrup.
Spread the sponge evenly with the chocolate custard then roll again and secure tight with cling film. Leave in the fridge for a few hours before serving.
Dust with icing sugar and slice as thick as you want!