For the moulds: 30 g beeswax and 30 g butter melted together (or just butter)
How to make it
Batter (15 min): Bring milk, butter and vanilla just to a simmer, then cool to warm (about 50C/120F). Whisk eggs, yolks and sugar. Whisk in the flour and salt, then pour in the warm milk slowly, whisking gently so you don't beat in air. Stir in the rum.
Rest (24 to 48 hr): Strain into a jug (scrape the vanilla seeds back in), cover and refrigerate at least 24 hours, ideally 48. This is what gives the custardy center and stops them puffing up.
Coat the moulds (5 min): Heat copper or silicone canelé moulds in a 120C oven, brush inside with the hot wax-butter mix, turn upside down so the excess drains, and chill. With silicone, butter alone works.
Bake (1 hr): Heat the oven to 230C (450F). Stir the batter gently and fill the moulds to 1 cm below the rim. Bake 15 minutes at 230C, then lower to 190C (375F) and bake 45 to 55 minutes more, until the canelés are very dark brown, almost black at the top.
Unmould (1 hr): Tip them out straight away onto a rack. They're crisp for a few hours only, so bake the day you eat them.
Tips
If they rise out of the mould like mushrooms, tap them back down and keep baking; the batter was too cold or not rested enough.