Yorkshire Pudding Batter -- Letting it "Rest"
I previously addressed this on a related question here. I cited Kenji Lopez-Alt's great article about Yorkshire puddings based on his testing a variety of recipes. For the full thing, you can find it on Serious Eats. As to resting and batter temperature, this is what he says:

Resting Batter:

This was what the tester determined made the largest difference in the quality of the finished product:


  I'm going to say this: Resting your batter is the single most important step you can take to improving Yorkshire pudding and popovers. Not only do they come out taller, they also come out much tastier, with a more complex, toasty flavor. Non-rested-batter puddings taste positively flat (literally and figuratively) next to rested-batter puddings.  
  
  I'd go so far as to say that resting at least overnight is essential if you are really after the best.


Batter temperature:


  
  Warmer batter will create taller, crisper puddings with a more hollow core
  Colder batter will create denser puddings with a more distinct cup.
  


So, definitely rest your batter (in the fridge!) and pick the batter temperature that matches the outcome you'd prefer.