Mexican cheese sauce

We have a mexican restaurant in town and they always bring us out chips and salsa; I love them. Sometimes we will order some Caso (sp?) [cheese] sauce to dip the chips in. All it is is cheese, not salsa in it. But it is creamy and really tasty. When it gets cold, it gets real thick. I'd love a recipe for it, if anyone knows how to make this and what kind of cheese it is. I asked at the restaurant and they told me "white cheese".

Thanks,
Kathy