Preheat oven to 350 F. Spray a Bundt pan with flour-added baking spray.
Mix together flour, sugar, cocoa, soda, baking powder and salt in large bowl.
Add oil, milk, eggs and vanilla. Beat two minutes with electric mixer at medium speed. Stir in water until blended. Batter will be thin.
Pour batter in the Bundt pan and bake for 45-50 minutes or until skewer or pick inserted comes out clean.
Let the cake sit in the pan for about 10 minutes, then carefully turn it from the Bundt.To make the frosting, sift the sugar and set it next to the stove. Keep the cocoa powder handy, too. I don’t usually mix the two ahead of time, because I know I’ll use all of the cocoa powder, but I don’t always use every bit of sugar.
Combine the butter, marshmallows and milk in a large (3 quart) saucepan.
Heat on medium to medium low, stirring often, until melted and smooth.
Remove from heat and stir in cocoa.
Add about half the powdered sugar and stir well, then gradually add more until you get just the consistency you want. I usually use it all.
Add vanilla and stir until smooth.
Pour over the cake, letting it fall down the sides and into the center.
