What is the clearest way to describe two "kitty-corner" buildings?

Both kitty-corner and catty-corner are corruptions of cater-corner (or catercorner); they are often used by people who indeed have never heard of the original expression. Others will have heard of the original expression and regard the corruptions as ignorant vulgarisms; still others will find them acceptable, albeit very informal. My own experience in giving directions to my building is that quite a lot of people even in the United States are not familiar with any of the words, and I have therefore resorted to saying diagonally opposite from or diagonally across from when I want my meaning to be understood at once. When I am hoping for a chance to give expression to my native didacticism, I say cater-corner.

I assume you are asking about a situation like this:

It is technically and mathematically correct to say that they are on opposite corners. However, this can indeed be misconstrued, as you've noted. I do not believe that there are any terms in common usage that would be guaranteed to be clear to any given audience.

If the buildings are diagonally across an intersection, kitty-corner has exactly the right connotation, but it should be clear even when talking about a single street. From Wiktionary:

(US, Canada) (with to:) located diagonally across from something, especially across an intersection