"Peanut butter" would be dealt with by including a reference under the "butter" entry. Something like:

'N, culinary. A paste made of ground up nuts, sometimes with additional oils and other ingredients. E.g. "peanut butter", "almond butter".'

"Amusement park", same. Falls very much under the "place of recreation" definition of "park".

"Black hole" is maybe a bit different, because it's a scientific term - and certainly in a science dictionary would be included as a two-word item - but, for consistency, in a regular dictionary should be handled identically to the above, with a note on the word "hole".

While including noun phrases as singular entities in a word game is entirely appropriate, I don't think the OP has formed a rigorous definition of the concept that they are trying to describe. I agree with the other comment which suggests that they need some instruction / practice using a dictionary.