Preheat oven to 350°F and line a 24-cup muffin tin with paper liners.
Combine graham cracker crumbs and melted butter, press about 1 tablespoon into the bottom of each liner, and bake for 5 minutes.
Prepare chocolate cake batter according to package directions and fill each cup two-thirds full over the graham crust.
Bake for 16 to 18 minutes until a toothpick comes out clean, then cool completely.
Beat butter until fluffy, add powdered sugar and marshmallow fluff, and beat until light and smooth, adding heavy cream as needed for consistency.
Pipe frosting onto cooled cupcakes and top with chocolate shavings and a pinch of crushed graham crackers.