PREHEAT oven to 350 degrees. Sift sugar and flour in a medium bowl. Add marshmallows. In a sauce pan, mix butter, oil, cocoa and Coca-cola. Bring to a boil and pour over dry ingredients;blend well. Dissolve baking soda in buttermilk just before adding to batter along with eggs and vanilla, mixing well.
POUR into a well greased 13 x 9 inch pan and bake 35-45 minutes. Remove from oven and frost immediately.
If you choose to use a bundt pan as I did you may need to cook up to 10 minutes more. Use a toothpick to test the middle. Make sure the toothpick comes out clean.
For Frosting: Combine butter, cocoa and Coca-cola in a saucepan. Bring to a boil and pour over confectioners sugar, blending well.
Add vanilla and pecans (if using pecans), spread over hot cake. When cool, cut into squares and serve.