Preheat oven to 350F and grease a 10 or 12-cup Bundt pan with cooking spray (I use Pam for Baking).
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 pan. Smooth the top with a spatula and whack the pan against the counter to evenly distribute.
Bake for 45-50mins or until a toothpick inserted into the center comes out mostly clean.
Place cake on wire rack to cool for 10mins. Whack the pan on the counter to loosen the cake 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. Add Baileys and stir until combined.**
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 Bundt has cooled completely, frost with the whipped ganache. Place in fridge to set (I let it sit overnight, but you just need it to firm up so you can add the marshmallow on top).
Frost with marshmallow fluff and torch to toast.
*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.