Preheat your oven to 350°F (175°C). Prepare an 8x8 inch baking pan by greasing it or lining it with parchment paper, leaving some overhang for easy removal.
In a microwave-safe bowl or on the stovetop, melt the unsalted butter until fully melted. Allow it to cool slightly.
In a large mixing bowl, whisk together the melted butter, brown sugar, and granulated sugar until well combined.
Whisk in the egg, followed by the egg yolk, and vanilla extract to the butter and sugar mixture. Mix well until smooth and creamy.
Using a rubber spatula, fold in the all-purpose flour, baking powder, salt, and cornstarch into the wet ingredients mixture until just combined. Do not overmix.
Gently fold in the graham cracker crumbs until evenly distributed throughout the batter.
Pour the blondie batter into the prepared baking pan. Use a spatula to spread it out evenly.
Place the pan in the preheated oven and bake for 23-25 minutes, or until the edges are golden brown and a toothpick inserted into the center comes out with a few moist crumbs attached.
Once baked, remove the pan from the oven and allow the blondies to cool completely in the pan. While the blondies are cooling, make the ganache.
FOR THE GANACHE
Place the cream in a medium microwave-safe bowl and heat until hot and slightly bubbly, ensuring it doesn't boil over. Start with 30-second intervals, checking until bubbly.
Remove from the microwave and add the chocolate. Let the mixture sit for 5 minutes without stirring. If the chocolate hasn't fully melted after 5 minutes, microwave again in short intervals until smooth.
Once ready, stir until everything melts and the mixture is smooth. Pour the ganache over the cooled blondies and spread evenly. Place in the fridge to firm up while making the marshmallow fluff.
FOR THE MARSHMALLOW FLUFF
In the bowl of a standing mixer, use a whisk to combine the egg whites and salt until frothy. Keep the mixer on the lowest speed to maintain stability.
In a small saucepan, combine the sugar, corn syrup, water, and vanilla over medium heat. Heat until the sugar is fully dissolved, reaching about 240°F (115°C) or the soft ball stage, about 3-5 minutes.
Once the sugar has melted, turn the mixer to medium-high speed and slowly pour the hot sugar mixture down the inside rim of the mixing bowl, letting it run down the side slowly. Pour very slowly to avoid scrambling the eggs. This process should take about 2 minutes.
Once all the sugar has been added, increase the mixer speed to high and beat for about 5 minutes or until the mixture is very stiff and shiny.
Gently spread the marshmallow fluff on top of the ganache layer. Using a kitchen torch, carefully toast the top of the marshmallow fluff until lightly browned. Ensure safety by keeping flammable materials away and maintaining a safe distance from the flame.