Preheat oven to 350°. Spray a 9″x 13″ baking pan with cooking spray.
Combine Coca-Cola and buttermilk. Set aside.
Whisk together flour, cocoa, and baking soda. Set aside.
Using an electric mixer on low speed, beat butter until creamy. Gradually add sugar and beat until blended. Add eggs and vanilla, and beat until blended.
Add flour mixture to butter mixture, alternating with Coca-Cola mixture. Begin and end with flour mixture. Beat at low speed just until blended after each addition. Stir in marshmallows.
Pour batter into prepared pan. Bake for 30-35 minutes. Cool for 10 minutes before frosting.
Place butter, Coca-Cola, and cocoa in a large saucepan. Bring to a boil over medium heat, stirring until butter is melted.
Remove from heat. Whisk in sugar and vanilla.
Immediately pour frosting over warm cake. (Do not make frosting ahead!) Sprinkle with pecans.