Is it okay to put baking powder in Yorkshire Pudding batter?
Yorkshire puddings rise because of the eggs in them. This means that the mixture for you Yorkshire puddings needs heat to rise So if your oven is not hot enough, they won't rise as much as you want. So here are some tips:

-make sure your oven is hot before putting your puddings in

-Don't open the oven while cooking your puddings

-I always pre heat the muffin trays before I use them.

So you put your mixture in a hot muffin tray (or something els you use), put the hot muffin tray in a hot oven, and don't open the oven anymore before the cooking time is over. So I won't use baking powder, I first try it by making sure the heat is there. 

Note that of course every oven is different and so you might need a couple of time before have the right temperature. Also I recommend this site for more information about Yorkshire puds: http://www.hub-uk.com/tallyrecip03/recipe0124.htm