Don't Don't buy aunt Bessies yorkshire puddings. They are the work of the devil, and not like the real thing at all.
Try this
4 oz plain flour
2 eggs
Milk
A splash of COLD water.
pinch of salt
Basically the same as a pancake batter.
Put flour salt and beaten eggs in a bowl, add the splash of cold water (say an egg-cup full), then add some milk (maybe half a cup). Whisk hard and add more milk if necessary, so the batter is runny, bearly coats the spoon, with lots of air bubbles in it.
Now put your oven on full, put some dripping (or veg oil if you must) into a yorkshire pudding tin (or an ordinary meat roasting tin if you want to make one big yorkshire).
Put that into the oven until the fat or oil is VERY hot. Then take the tin onto your hob with heat under it, so the fat stays very hot (almost smoking in fact). Pour in the batter, which should sizzle as it goes in. This ensures your puds rise.
Quickly back into your hot oven, for about 20 mins, not much longer. Yorkshire pud should rise well round the edges but should retain a slight spongeyness in the middle, which is why it is called 'pudding', and means it will soak up some gravy/meat juices on your plate.
Aunt Bessies, and most restaurant yorkshire puds are generally over cooked and crunchy,without that softer bit in the middle.
Sorry if the instructions sound complicated. It's actually really easy to make. Most people just don't get the fat hot enough, which is why they don't rise.