I've tried to make canele for several times. first time it looks great. but after that, it always pop up so high, expanded, and stuck. I always baked at 180C/180C. maybe it's too low and I'm going to try 200 or 250C later.
Try pouring less batter in the mold. And baking around 180C should be fine.
why to chill the batter over night? what will happen if i don't chill it?
and when it comes out of the fridge, should I steer and mix the batter well. because there is a thick layer of butter floating on the batter.
Chilling the batter helps the flour to hydrate thoroughly. Just whisk the batter well before you start pouring (and if you are doing a lot, whisk it occasionally as you continue to pour). You can leave the batter out for an hour or so before you want to bake. This will let the butter chunks soften up some so they are easier to whisk in; otherwise you risk having them floating on top after you've poured the cannelés.