The words "[it is] pancakes" are incompatible because "are" is the plural equivalent of "is".
Not true. "It is pancakes in a can" is entirely grammatically correct, because "pancakes in a can" is a singular noun -- an idiomatic one, to be sure, but "pancakes in a can" is taken to mean "a can containing pancakes", which is obviously singular.
Not true. "It is pancakes in a can" is entirely grammatically correct, because "pancakes in a can" is a singular noun -- an idiomatic one, to be sure, but "pancakes in a can" is taken to mean "a can containing pancakes", which is obviously singular.