Preheat oven to 350 degrees. Coat pan with baking spray.
Melt butter, cocoa and cola together in saucepan and bring to a boil, stirring often
combine flour, sugar and soda in a big mixing bowl
stir hot cocoa mixture into the flour mixture until well moistened
add marshmallows and buttermilk and continue stirring
add eggs and vanilla and stir until all is well blended.
pour into prepared pan and bake at 350 degrees for approximately 35 minutes until wooden pick inserted comes out clean.
spread icing on top of warm cake and cover with chopped pecans