In a saucepan, combine the whole milk, vanilla bean and seeds, dark rum, and butter. Heat the mixture until it just comes to a boil. Then remove it from the heat, cover the pan, and let it steep for 20 minutes.

In a large bowl, whisk the eggs and sugar for about 1 minute until the mixture looks pale. Next, whisk in the flour until the batter is smooth. After that, pour in the warm milk mixture in two additions, mixing well after each one.

Cover the bowl with plastic wrap and refrigerate the batter overnight, or for at least 12 hours.

Preheat the oven to 480°F (250°C). Grease the molds very well with cooking spray or melted butter.

Stir the cold batter gently, taking care not to whip air into it. Fill each mold about three-quarters full with batter.

Bake the canelés at 480°F for 10 minutes. Without opening the oven door, lower the temperature to 350°F (175°C) and bake for 1 hour more.

Let the canelés cool in the molds for 5 minutes, then turn them out onto a rack. Serve them the day they are baked for the best crisp texture.