I'm writing this letter to you on this very special day. I know you're rushing around, trying to get ready, so I'll be quick.
Have fun today. You look beautiful and happy. Don't be nervous. You two have many, many years together ahead of you. It won't always be easy, and yes, you'll do some stupid things, but you'll get through everything together. Always remember to kiss each other goodbye in the morning. Hug often, and say "I love you" frequently. You know it, but it's always nice to hear it said. Laugh together often. You are about to embark on a grand adventure with your best friend.
P.S. He was right, chocolate cake was an excellent choice for the wedding cake.