Here is the item again:
Hands covered in cookie dough, (A)
Madeline ran to the phone, (B)
hoping that the call was from Eric.
(C) Her boyfriend who
had broken his promise to call the day before.
You correctly realized that the period after Eric was wrong. Her boyfriend who had broken ... is an appositive. Wherever you attach an appositive—at the beginning, in the middle, or at the end of a sentence—you must use appropriate punctuation.
To fix the problem, you chose to do this:
Hands covered in cookie dough, Madeline ran to the phone, hoping that the
call was from Eric, her boyfriend who had
broken his promise to call the day before.
You have correctly employed Punctuation Rule 9:
Main clause + , + appositive.
Nice job!