Warm the whole milk with the split vanilla bean in a 2-quart / 2 L saucepan over medium heat until steaming, about 5 minutes. Don't let it boil.

Meanwhile, whisk the egg yolks and sugar in a mixing bowl until pale and thick, about 2 minutes. Whisk in the cornstarch and salt until smooth.

Remove the milk from heat and slowly pour a ladleful into the egg mixture, whisking constantly to temper. Pour the tempered eggs back into the saucepan with the rest of the milk.

Cook over medium-low heat, stirring constantly, for 5 to 7 minutes until the custard thickens and coats the back of a spoon at around 82C/180F. Pull it off the heat as soon as it thickens, it can scramble fast if it keeps cooking.

Divide the custard evenly between two bowls. Stir the cocoa powder and melted dark chocolate into one bowl until smooth, that's your chocolate custard. Leave the other bowl as is for vanilla custard.

Press plastic wrap directly onto the surface of both custards to stop a skin forming. Chill for 20 to 30 minutes, until just warm to the touch.

Pour the Alchermes (or dark rum, water, and grenadine substitute) into a shallow bowl.

Dip each ladyfinger into the liquid for 1 to 2 seconds per side, just enough to soak through without going soft. Arrange a layer in the bottom of a 2.5-quart / 2.5 L glass trifle dish.

Spread half the vanilla custard over the ladyfingers. Add another layer of soaked ladyfingers, then spread half the chocolate custard on top.

Repeat with the remaining ladyfingers, then finish with the last of the vanilla and chocolate custard, swirling the two together slightly on top if you like a marbled look.

Cover and chill for at least 4 hours, or overnight, until the layers are fully set and slice cleanly.

Shave the remaining dark chocolate over the top just before serving.