Preheat oven to 350 degrees. Grease a 9 x 13 baking dish.
In a mixing bowl, beat cake ingredients until smooth.
Pour batter into prepared pan. Bake for 25-30 minutes or until toothpick inserted comes out clean.
Remove cake from oven and immediately sprinkle top with mini marshmallows.
FROSTING:
In a saucepan, over medium heat, melt butter. Whisk in sugar and evaporated milk. Let mixture come to a boil and let it boil for approx 30 seconds. Whisking frequently.
Remove from heat and whisk in chocolate chips and vanilla until smooth and completely melted. Quickly pour over marshmallows and smooth evenly.