Preheat the oven to 350 degrees.
Lightly grease a 9x13 baking dish.
In a large mixing bowl combine the dry ingredients: flour, sugars, cocoa powder, baking powder, baking soda and salt.
Add oil, eggs, vanilla, milk and sour cream. Mix well.
Carefully add the boiling water and mix.
Pour batter into the prepared pan.
Bake for 30 minutes.
Remove cake from oven and sprinkle marshmallows all over, completely covering it.
Return to the oven and bake for 3-5 minutes. Allow the cake to cool completely before drizzling frosting on top.