Take another look at the sentence:
Slathering the popcorn with melted butter, the calorie count skyrocketed.
Slathering the popcorn with melted butter, a participle phrase, has no target in the sentence. When a modifier has no word to describe, it is dangling.
To fix the problem, you should revise the sentence like this: When Aida decided to slather the popcorn with melted butter, the calorie count skyrocketed. Adding Aida to the sentence lets the reader know who is ruining the calorie count.