Mix red velvet cake mix according to box instructions. Substitute the water for milk.
Coat waffle maker with cooking spray. Fill waffle maker with designated amount of batter and bake according to waffle cooking instructions. Allow to cool slightly.
Prepare the marshmallow fluff by microwaving the butter for 10-15 seconds just until softened.
Combine the softened butter and marshmallow fluff in a mixing bowl and beat on medium-high speed until completely smooth.
Add powdered sugar and mix on medium speed. Fluff will become very thick. Add heavy whipping cream and vanilla extract and increase speed to medium-high. Whip for a couple minutes until smooth.
If fluff is too thick, add additional heavy whipping cream 1 teaspoon at a time. If too thin, add additional powdered sugar.
Assemble the stacks with a waffle at the bottom topped with a dollop of marshmallow fluff. Crush a quarter of a graham cracker and drizzle with hot fudge sauce. Repeat 3 times. Top with chocolate shavings if desired.