Spray a 9x13 inch baking dish with non-stick spray and set aside.
Prepare the cake, according to box directions.
After baking, allow the cake to cool completely.
Use a wide straw or the skinny end of a wooden spoon to poke holes throughout the cake.
In a mixing bowl, whisk together the pudding mix and milk, until smooth.
Quickly pour the pudding over the cake, aiming for the poked holes.
Smooth the pudding around to fill the holes in the cake, and to smooth evenly over the top.
Move this to the refrigerator to chill for 2 hours.
For the frosting, beat the butter 1-2 minutes.
Add the marshmallow fluff, vanilla, and salt. Mix until well combined.
Add the powdered sugar, and mix until combined.
Drop the frosting by spoonfuls over the cake, and smooth out evenly.
Sprinkle the mini marshmallows over the top of the cake.