Preheat the oven to 345℉. Lightly grease an 8x8 square pan and line it with parchment paper.
In a medium bowl, combine the graham cracker crumbs, flour, baking soda, and salt.
Add the egg and brown sugar to a stand mixer bowl. With the paddle attachment, beat the egg and sugar until combined, approximately 3 minutes.
Add the butter cubes to the egg and sugar mix. Beat until combined, about 5 minutes. Add the sweetened condensed milk and vanilla extract. Continue to beat until the mixture is light and fluffy, about 10 minutes.
Add the dry ingredients and mix on first speed just until combined.
Pour the batter into the prepared pan and spread out evenly. Bake for about 18 minutes. The middle will be slightly jiggly but set and the outer edges are golden brown. Allow to cool to room temperature.
Dark Chocolate Ganache
Using a double boiler, melt the chocolate and sweetened condensed milk. Stir together until smooth.
Pour the ganache over the top of the blondies and smooth out. Let set in the refrigerator while making the meringue.
Vanilla Bean Marshmallow Fluff
In the bowl of a stand mixer, combine the egg whites and sugar. Using a double boiler, heat the mixture to 160℉ while whisking constantly.
Attach the bowl to the stand mixer and add the whisk attachment. Whisk on medium speed for 3 minutes. Add the vanilla bean paste and a pinch of salt. Continue whisking on high speed for an additional 10 minutes until stiff peaks.
Scoop onto the chilled ganache and spread out. Make a swirl design with an offset spatula.
Torch the meringue with a kitchen torch. Be careful!
Notes
Store s'mores blondies in an air-tight container in the fridge for up to 5 days.