Since Dean loves food, Cas tries to cook for him. He always watches Dean take the first bite with little kid anticipation like he's the judge on a cooking show. Dean doesn't have the heart to tell him it's awful every time. So, he swigs down each bite with a sip of beer, a thumbs up and a nod because he'd eat the rubber from a bike tire if it meant that Cas was HERE every day, giving off his proud smiles for making Dean happy. And Dean is. So happy ♡. (You said you wanted a fluffy head canon :))
OMG I LOVE THIS SO MUCH!!
Then when Cas becomes human he tastes his own food and realises how bad it is and how Dean was sparing his feelings and he is so touched by it but also annoyed so he wants to learn to cook better.
He asks Dean to help and he teaches him little by little every day until one day Cas surprises Dean and Sam with an amazing meal that he managed to do all on his own (well, he did text Sam at one point to just check whether it was a bulb or a clove of garlic he needed, but that’s all) and they are all so happy and Cas is so happy and proud of himself, while Dean is proud of himself for teaching him so well and Sam is happy that they are all content.
Dean and Cas do the washing and tidying up in sync while Sam gets the movie ready then they all settle down to watch classic Marilyn Monroe with toffee popcorn for Dean and a whole heap of candy for Cas and Sam (skittles are Cas’ favourite and Sam loves gummy bears).