Preheat oven to 350°F. Grease a 9x13-inch baking pan with nonstick spray or line with parchment and set aside.
In the bowl of a stand mixer fitted with the paddle attachment, beat the butter, brown sugar, and sugar together on medium-high speed for 1 minute. Add in the egg, egg yolk, and vanilla, and continue to beat on medium high for another 1 minute.
½ cup unsalted butter, ½ cup brown sugar, ½ cup granulated sugar, 1 large egg, 1 large egg yolk, 1 teaspoon pure vanilla extract
In a separate bowl, whisk the flour, salt, and baking soda together. Add into the wet mixture and mix until a dough comes together.
2 cups all-purpose flour, 1 teaspoon kosher salt, ¼ teaspoon baking soda
Spread the cookie dough into the bottom of the prepared baking pan. Bake for 10 minutes, or until the edges of the dough start to turn golden brown. The center will not be completely firm - do not worry, it will firm up.
Place the chocolate bars on top of the baked cookie. Sprinkle the marshmallows on top of the chocolate. Place back into the oven for another 5 minutes, or until the marshmallows around the edges start to brown.
3 giant (7.56-ounce) Hershey’s chocolate bars, 3 cups mini marshmallows
While the marshmallows are browning, make the graham cracker crumble. Heat-treat the flour by adding it to a microwave-safe bowl and microwaving in 30-second increments until the internal temperature reaches 165°F. In a medium bowl, mix together the graham crackers, flour, and salt. Mix in the butter until a crumble has formed.
1 cup crushed graham crackers, ¼ cup heat-treated all-purpose flour, ⅛ teaspoon kosher salt, ½ cup unsalted butter
Once the marshmallows have browned, remove the bars from the oven. Sprinkle the graham cracker crumble over the surface of the bars. Cut into 12 bars and serve!