Preheat oven to 350 F. Grease a 9x13-inch baking dish.
In a large bowl, stir together flour, sugar, graham cracker crumbs, cocoa, baking powder, baking soda, and salt.
Make a well in the center of the dry ingredients.
Add buttermilk, oil, eggs, and vanilla.
Stir until just combined.
Add chocolate chips, and mix until just incorporated.
Pour into prepared baking pan.
Bake for about 20-25 minutes, or until a toothpick inserted in the center comes out clean.
Remove from oven, and let the cake cool for about 5 minutes.
Once you remove the cake from the oven, turn the oven to "Broil".
Topping
Meanwhile, warm marshmallow creme until spreadable (about 30 seconds in the microwave).
Pour creme evenly over cake, and gently spread to cover the warm cake
Place marshmallows evenly over the top of the cake.
Sprinkle remaining chocolate chips in between marshmallows.
Return cake to oven, and broil until marshmallows are puffed and golden. Watch carefully because the marshmallows can burn quickly.
Remove from oven.
Serve warm or at room temperature, sprinkling with extra graham cracker crumbs just before serving.
Notes
All-purpose flour: It's important to properly measure the flour. Weighing the flour is the most accurate way to measure it. You can also sift or stir it to break it up, lightly spoon it into the measuring cup, and level. Too much flour will give you a heavy, dry cake.
Graham cracker crumbs: You can either use store-bought graham cracker crumbs or crush about 3 full sheets of graham crackers.
Buttermilk: I like to use full-fat buttermilk in this cake. If you don't have buttermilk, you can make a homemade buttermilk substitute.
Marshmallow crème: Marshmallow crème is also called marshmallow fluff.
Marshmallows: I use large marshmallows, but mini marshmallows could also be used. If using smaller marshmallows, check on them sooner as they may toast more quickly.