(This is a lot of people ._. I’ll only do this one because I didn’t clarify but not this many characters next time please you guys. I hope you don’t mind that it’s general)
Monkey D. Luffy: He would run over and try to take a cookie the minute he saw them, asking his partner if he could help and that he liked their voice.
Roronoa Zoro: He’d lean against the doorway and watch them for a bit, blushing a bit at the lyrics and at their voice. He’d walk over and snatch a cookie and complimenting their voice.
Nami: She’d immediately go over and help them with decorating or with baking and urge them to continue singing. She would sing along with them if she knew the song and if not, she would hum along to them.
Usopp: He’d blush once he understood the lyrics that they were singing at their voice. He would shyly walk over and offer them help, taking a cookie.
Sanji: He’d help with the baking as soon as he walked into the kitchen. He’d smile at their voice and sneak in a cheeky comment about being their ‘Santa Baby’.
Tony Tony Chopper: He’d hurry over and hug their leg, asking if he could help. He’d kiss their cheek and compliment their singing.
Nico Robin: She’d lean against the doorway and watch her partner for a while with a small smile on her face. She’d go unnoticed and keep their voice to herself, not letting them know she heard them.
Franky: He’d help decorate with them and listen to their voice, head swaying a bit if he liked the song. He now knows that he likes his partner’s singing voice.
Brook: He’d stay by the door for a bit to listen to their voice. He’d make himself known and compliment his partner’s singing voice, asking to duet next time.
Portgas D. Ace: He’d be helping as much as he could once he entered the kitchen. He’d hum along to his partner’s voice and try to focus on the cookies so he doesn’t accidentally burn the cookies in his hand.
Sabo: He’d smile and compliment his partner for their voice, blushing lightly. He would help them decorate and continue to listen to their voice
Buggy the Clown: He’d peak into the kitchen and blush at his partner’s voice. He wouldn’t make himself seen as he would be too embarrassed and wouldn’t let anyone know that he actually loves their singing voice.
Shanks: He would help with all he could, probably making a mess in the process. He would probably ask his partner to teach him the song and he would sing with them since it was fun.
Dracule Mihawk: Leaning against the doorway, he’d watch his partner bake and decorate. He’d listen to them and have to admit to himself that he liked it. He would wait for his partner to notice him and smirk at their surprise.
Coby: He’d blush instantly at hearing their voice and would feel himself fall for them even more. He’d speak up and ask them if they needed help and would help all he could, still blushing.