What kind of butter is used for roast beef skewers?

You mean butter or beef? If you want meat, use the brain skin, which is the layer near the ribs on the thick loin. Most meat sellers know it, but some people don't want to cut it for you. The meat here has tendons in it. It's delicious and tender.

If it's oil. I've never heard of kebabs made of butter. When they get cold, they stick to their mouths. My family used chicken oil mainly based on soybean oil when opening a shop.