Elsa doesn't need to bring a dish to dinner, but it's tradition and she she wants to share some Norwegian cuisine. Anna eyes her skeptically until she agrees to bring sweets, not fish heads or whatever they ate. It gets a little chuckle out of Elsa, which just sets Anna's mood for the rest of the day.
Even when the humour has passed, Anna still has a really stupid smile on her face. She knows she does, she can feel it. But, it's too good to hide away. Especially when Elsa smiles back, soft and warm.