Does butter + yogurt = sour cream??? in a pinch

I would omit the butter. Yogurt by itself is a decent choice for a substitute.

Could you tell us what you are making? A baked item? Or something cooked? I sometimes use yogurt as a substitute for sour cream, but it would depend on the recipe.

I use the same amount of whole milk plain greek yogurt in the place of the sour cream in recipes. I don't know about butter, but hope this helped!