Here is the sentence:
Slathering the popcorn with melted butter, the calorie count skyrocketed.
Good choice! You recognized that the modifier is dangling. Slathering the popcorn with melted butter, a participle phrase, has no target in the sentence to describe.
To fix the problem, you could 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.