Preheat the oven to 350°F. Grease and flour a 9x13; set aside.
Combine buttermilk, sugar, brown sugar, vegetable oil, eggs, and vanilla with an electric mixer.
In a separate bowl, whisk together flour, baking soda, salt, cinnamon, and nutmeg. Slowly add flour mixture to the buttermilk mixture. Fold in carrot, pineapple, and pecans.
Pour cake batter into prepared 9x13 pan. Bake 30-35 minutes or until a toothpick inserted comes out clean.
Prepare frosting by beating cream cheese and butter together in a medium bowl with an electric hand mixer; add vanilla. Add powdered sugar a half cup at a time. Spread frosting over cooled cake and top with extra pecans, if desired.