영어 지문에서 one과 it이 가리키는 것

다음 문장에서 one과 it이 각각 무엇을 가리키는지 궁금합니다.
“She soon returned with two mugs of coffee and two pieces of mouth-watering chocolate cake. Confused, I looked at her as she sat down across from me. She was smiling and I felt my dark mood slowly lifting. ‘This one is free. I thought you might need it,’ she said.”