Preheat oven to 325 degrees. Line cupcake pans with liners (24 cupcakes). Set aside 1/4 cup of the crushed graham crackers.
In a small bowl combine remaining crushed graham crackers, melted butter, and sugar. Spoon about 1 tbsp of cracker mixture into each cupcake liner, press down. Bake for 5 minutes, remove from oven and turn up the oven to 350 degrees.
Prepare boxed chocolate cake mix batter according to package directions. Scoop batter evenly into cupcake liners. Bake according to package instructions. Cool completely on a cooling rack.
If using a stand mixer, chill the whisk attachment and bowl in the freezer for about 5 minutes. If using hand beaters you can also chill those and the bowl you are using. This just helps the whipped cream come together better.
Pour the heavy whipping cream into the chilled bowl. Beat on med-high for about 3-5 minutes or until stiff peaks form. If you can tip the bowl upside down and the whipped cream doesn't move, you're good. Spoon the marshmallow fluff into the whipped cream, and add the vanilla extract. Beat on medium speed just until combined. Chill until ready to frost cupcakes.
Frost the cupcakes with either a knife or piping bag. I use a gallon size plastic zip bag and cut the corner. Sprinkle with reserved crushed graham crackers. Top with 3 (or more) mini marshmallows on each cupcake. Drizzle with chocolate fudge. If fudge is too thick and cold, microwave for a few seconds but be careful not to overheat or it will melt the whipped cream.
Store in air tight container in the refrigerator.
If you are decorating without a piping bag/zip top bag you'll likely use less frosting per cupcake, so you may want to reduce the amount of heavy cream to 2 cups.