It usually isn't roasted (just melted to render it out), and has no flavoring. You'd have to fix it up. You might do as well or better to make something like shortribs or untrimmed brisket that have a lot of drippings, and freeze them for this kind of thing. Many people do. They save the fond and fat and use it later. If you want something even richer, you could roast the heck out of some soup bones or knuckles and make stock from the bones, saving the drippings for your Yorkshires.

Use butter.....Tallow will be nasty in my opinion. Or you could buy a chunk of suet and render it out to cracklings. It would be tallow but might have some flavor besides just tallow. But butter is always good.

Yorkshire puddings need very high heat. The fat must be sizzling when the batter goes into the tin. Butter would burn. Use lard, dripping or oil. Duck and goose fat are good too. https://www.theguardian.com/lifeandstyle/wordofmouth/2012/feb/16/how-make-perfect-yorkshire-puddings

Can't you use lard?

I coulld use lard, ut I was hoping to get some semblance of beef tasting.

I just ask the butcher for some fat previously trimmed (or waiting to be trimmed) from other roasts. Or to not trim mine when I order it. Barring that, I use suet.

It won't make enough difference to the flavour. After all, you eat the pudding with roast beef, gravy and veg. Flora's right in her choices.

I've made them with butter.....hot HOT pan, pat of butter then batter and into the oven....the butter browns but doesn't burn.

i hear that is a hanging offence.