Preheat oven to 350F and grease and flour three 6" cake rounds, line with parchment.
In a medium bowl, whisk flour, graham cracker crumbs, baking powder, baking soda, and salt until well combined.
Using a stand mixer fitted with the paddle attachment, beat butter until smooth.
Add sugars and beat on med-high until pale and fluffy (2-3mins).
Reduce speed and add eggs one at a time, fully incorporating after each addition. Add vanilla.
Alternate adding flour mixture and buttermilk, beginning and ending with flour (3 additions of flour and 2 of milk). Fully incorporating after each addition. Do not overmix.
Spread batter evenly into prepared pans. Smooth the top with a spatula and whack the pans against the counter to evenly distribute.
Bake for approx. 40mins or until a toothpick inserted into the center comes out mostly clean.
Place cakes on wire rack to cool for 10mins. Whack the pans on the counter to loosen the cakes and turn out onto wire rack to cool completely.
Milk Chocolate Ganache:
Finely chop chocolate and place into a bowl. Bring cream just barely to a simmer and pour over chopped chocolate. Cover with plastic wrap and let stand 2 mins. Stir with a spatula until combined and smooth. **
Cool in the refrigerator until thickened, but still soft (stirring occasionally). ***
Transfer to bowl of stand mixer and whip using the whisk attachment until the ganache is fluffy and has lightened in color (approx. 1 or 2 minutes).
Once the cakes have cooled completely, frost the layers with the whipped ganache. Place in fridge to set.
Spread marshmallow fluff roughly all over cake and torch to toast.
Serve at room temperature.
*You can use a milk chocolate buttercream if you prefer. **If there are still some chunks of unmelted chocolate, microwave in 10 second bursts until all chocolate has melted. ***You can make the ganache a day ahead and refrigerate it overnight, then take it out and allow it to come to room temperature before whipping.