Preheat oven to 350 degrees. Lightly spray a 9 x 13" cake pan with non-stick baking spray. Set pan aside.

In mixing bowl, combine flour, sugar and salt. Set aside.

In saucepan, add butter, oil, cocoa powder, and water. Over medium heat melt the ingredients together stirring often. Once everything has melted together, remove from heat. Pour the hot chocolate liquid over the dry ingredients. Mix together with a large whisk or hand mixer.

Dissolve baking soda in the buttermilk. Stir together until the buttermilk becomes foamy. Pour the buttermilk into the cake batter mixture. Add in the eggs and vanilla. Whisk or mix together until all ingredients are well incorporated and smooth. Pour the batter into the prepared pan.

Bake for 30-35 minutes. To test for doneness, insert a toothpick into the center of the cake. If the toothpick comes out clean and crumb-free, the cake is done. Let cake cool to room temperature before frosting.

For frosting: In medium saucepan, heat the milk, sugar, and butter stirring often. Promptly remove from heat after mixture comes to a simmer. Add in chocolate chips and marshmallows, and beat until both are melted and smooth. Let cool for about 1-2 minutes, while stirring often. Pour the frosting over the cooled cake. Let the frosting set up and cool at least 1 hour before slicing the cake.