You'll need a special mold for this canelé recipe.
- 2 tablespoons (1/4 stick) unsalted butter, diced, plus more, melted, for brushing
- 1 vanilla bean, split lengthwise
- 3/4 cup all-purpose flour
- 3 large eggs, beaten to blend
Combine milk and 2 Tbsp. butter in a medium saucepan. Scrape in seeds from vanilla bean; add bean. Bring to a boil; remove from heat. Cover and let stand for 10 minutes.
Whisk sugar, flour, and salt in a large bowl; set aside.
Strain milk mixture into a medium bowl; discard vanilla bean. Gradually whisk in eggs to combine. Gently whisk into flour mixture until just blended. Whisk in rum. Strain batter through a fine-mesh sieve into another medium bowl, pressing on any clumps of flour to push through.
Let batter cool completely, uncovered. Cover and chill for at least 24 hours and up to 48 hours (the flavor after 48 hours is best).
Arrange a rack in middle of oven and preheat to 450°. Lightly brush canelé molds with melted butter. Gently stir batter with a spatula (batter will separate). Fill prepared molds 2/3 of the way with batter, reserving half of batter for second batch.
Bake canelés for 20 minutes. Lower oven temperature to 400° and bake, tenting molds with foil if canelés are browning too fast, until deep mahogany brown, 45–50 minutes longer.
Remove canelés from oven and unmold onto a wire rack; let cool. Let molds cool, brush with more melted butter, and repeat with remaining batter. DO AHEAD: Can be made 6 hours ahead. Let stand uncovered at room temperature.