Preheat oven to 350 degrees. In a bowl, sift 2 cups sugar and 2 cups flour. Add marshmallows. In saucepan, mix 1/2 C butter, 1/2 C oil, 3 Tbsp cocoa, and 1 Cup 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 1 tsp vanilla extract, mixing well. Pour into a well-greased 9-by-13-inch pan and bake 35 to 45 minutes. Remove from oven and frost immediately.

Frosting:

To make frosting, combine 1/2 C butter, 3 tbsp cocoa and 6 tbsp Coca-Cola in a saucepan. Bring to a boil and pour over confectioners'' sugar, blending well. Add 1 tsp vanilla extract and pecans. Spread over hot cake. When cool