Cut the butter into small pieces and mix with the milk in a large saucepan. Split the vanilla bean in 2 pieces, scrape out the vanilla and add them to the saucepan. Set the saucepan over low heat and cook, stirring regularly with a wooden spoon, until the butter has melted and the milk starts bubbling around the edges (it shouldn't boil at all).
Place the flour, sugar, egg and yolk in a large bowl and mix together, stirring thoroughly before adding each ingredient. Pour in the hot milk mixture slowly, gently stirring with a large silicone spatula. Let the dough cool to room temperature, about 10 minutes. Chill in the refrigerator for 24 hours.
Preheat the oven to 475 degrees F. Spray the interior of each canele mold with butter spray, then set the molds in the freezer for 5 minutes to harden the butter.
If you choose to flavor your caneles, add some lemon, orange or lime zest to the dough at this time.
Fill the canele molds 3/4 of the way, leaving a 1/5-inch space at the top of each cavity. Bake the caneles for 4 minutes, then lower the oven temperature to 360 degrees F and bake until golden brown, about 56 minutes more. Remove the molds from the oven and immediately turn them over onto a cooling rack. Let them cool at room temperature for 10 to 15 minutes.
This recipe was provided by a chef, restaurant or culinary professional. It has not been tested for home use.