Answer: Gouda cheese
When Viola (as Sebastian) is teaching Duke how to make conversation with girls, she acts as herself, and after Duke asks her what her favourite cheese is, she says it's Gouda. When Duke tries the line on Olivia, however, it doesn't go as well.
After Viola reveals her true identity at the Cornwall game, her and Duke's relationship is strained, since she has been lying the entire time. She gives the real Sebastian the Gouda to give to Duke, which comes with an invitation to the ball. Duke forgives Viola and attends as her date.