Preheat the oven to 325°. Spray a 9x13 pan with cooking spray and set aside.
In a large bowl whisk together the vegetable oil, warm coffee, eggs, and vanilla until combined.
In a medium bowl sift together the flour, sugar, unsweetened cocoa powder, baking soda, and salt. Add the dry ingredients to the wet ingredients and gently whisk together until fully combined.
Fill the pan and bake for 40-45 minutes, until the center of the cake springs back lightly when pressed on, or a toothpick comes out clean. Immediately add the whole bag of marshmallows over the top of the hot cake and return to the oven. Bake for 2-3 minutes, just until the marshmallows are starting to puff and melt.
Remove the cake from the oven and allow it to cool completely. Drizzle the frosting over the cake while the frosting is still warm. Allow to chill in the fridge for about 20-30 minutes until the frosting has set before serving.
In a medium saucepan over medium heat combine the butter, milk, and cocoa powder. Cook until simmering and simmer for 2-3 minutes, being sure to stir constantly.
Remove from the heat and whisk in the vanilla and powdered sugar. You can use a hand mixer to get the frosting nice and smooth if you would like.
Drizzle the warm frosting over the cake and allow to chill in the fridge for 20-30 minutes before serving.