1️⃣ In a small saucepan, heat milk & butter until just steaming. Set aside to cool slightly.
2️⃣ In a bowl, whisk sugar, flour & salt. In another bowl, whisk eggs & yolks. Combine eggs with dry ingredients.
3️⃣ Slowly whisk in warm milk mixture, a little at a time, until smooth. Stir in rum & vanilla. Batter will be thin.
4️⃣ Cover and chill batter overnight (or at least 12 hours).
5️⃣ Preheat oven to 450°F (230°C). Grease canelé molds generously with softened butter (and beeswax if you want super-traditional shine). Place molds on a baking sheet.
6️⃣ Fill each mold about 3/4 full with chilled batter. Bake 15 minutes at 450°F, then lower to 375°F (190°C) and bake another 45–50 minutes until deeply golden brown.
7️⃣ Cool 10 minutes, then carefully unmold. Serve warm or room temp!