Step 1

In a mixing bowl, place cake mix, water, oil, and eggs; beat until well combined. Pour batter into a greased 9x13-inch baking pan.

Step 2

Bake at 350 degrees for 30-35 minutes, or until cake tests done with a wooden pick.

Step 3

Remove cake from oven and immediately pour marshmallows evenly over the top. Set aside.

Step 4

Place butter, sugar, and evaporated milk in a medium-sized saucepan.

Step 5

Bring to a boil over medium to medium-high heat, stirring frequently; boil for 30 seconds.

Step 6

Remove from heat; add chocolate chips and vanilla extract. Whisk until chocolate chips are melted and icing is thickened and glossy.

Step 7

Working quickly, pour over marshmallows and smooth with a rubber spatula.

Step 8

Allow to cool completely.