Preheat oven to 350 degrees F and grease a 9x13 baking sheet.
In a large bowl, combine the buttermilk, pineapple, shredded coconut, eggs, vegetable oil, and sugar.
Add the flour, baking soda, salt, and cinnamon.
Fold in the shredded carrots.
Pour the batter into the greased baking sheet. Bake for 45-48 minutes.
To make the frosting, mix the butter, cream cheese, and powdered sugar with an electric mixer. Mix for 4-5 minutes or until fluffy.
Allow the cake to cool completely before frosting. Decorate with chopped walnuts, if desired.