Put a cup of coca cola, butter, vegetable oil, and cocoa in a saucepan. Bring to a boil, Set aside for a few minutes.
Combine the flour, sugar and baking soda in a mixer bowl. Whisk to combine. Add the buttermilk and the cocoa mixture to the flour and beat until thoroughly combined. Add the eggs and mix well.
Pour into the prepared pan. Spread out to the edges. Place pan in center of oven. Bake for 25 - 30 minutes.
When the cake comes out of the oven, make the frosting.
Put the butter, cocoa, and milk in a saucepan. bring to a boil and boil for one minute. Remove from heat and add confectioner's sugar. Spread on top of cake. Add walnuts if using.
Recipe by Recipes Food and Cooking at https://recipesfoodandcooking.com/2015/04/18/fudgey-coca-cola-cake/