The first step is to prepare the pastry cream. To do this, bring the milk to a very low boil with the vanilla bean and a bit of lemon zest.
While the milk is coming to heat mix the yolks with 350 g of sugar in a large bowl, once mixed well add the flour.
Strain the milk into the bowl and whisk quickly, then pour back into the pan the milk was in and return to low heat. Continue to stir until the consistency of the pastry cream is correct. This will take 5 or 6 minutes.
Remove the pastry cream from heat and separate into two bowls. In one sprinkle a bit of sugar and then cover. In the other add the cocoa powder and mix until smooth and then cover. Let cool.
Make a syrup using 170-180 ml of water and the remaining 80 g of sugar. Add the Alchermes and pour into a flat baking dish.
Dip the ladyfingers (or sponge cake) on both sides to sop up color and flavor from the Alchermes syrup (approx. 2 seconds per side) and then set aside.
Place a base layer of ladyfingers into a glass bowl (or cylinder if available for presentation) and then cover with a portion of the cocoa pastry cream.
Add another layer of ladyfingers and then cover with the vanilla pastry cream. Continue to alternate layers of ladyfingers and pastry cream until you fill the container. Placing the ladyfingers perpendicular to the previous layer can help with stability.
Seal with plastic wrap and put to chill in the refrigerator for around 2 hours.
It is now ready to serve, top with whipped cream to taste or preference!
Ladyfingers are used as they are the most common form of this dish today, though a layer of sponge cake cut to the shape of the serving dish can be swapped in for the layers of ladyfingers when building the dessert. Quantities can be reduced by ratio to make a smaller amount to serve fewer guests.