Preheat oven to 350℉ and prepare pans.
In a bowl, add 2 cups flour and 2 cups sugar. Blend together and sift. Set aside.
In a medium pot, add 2 sticks margarine, 1½ cups marshmallows, and 3 tablespoons cocoa. Cook over medium heat until marshmallows melt. Pour over flour mixture.
Add to flour mixture 2 beaten eggs. Mix well.
In a bowl, add 1 cup coke, ½ cup buttermilk, and 1 teaspoon vanilla. Combine well.
Pour into prepared pans and bake for 40 to 45 minutes, or until toothpick inserted in middle comes out clean.