1. For the graham cracker/biscuit brownie and marshmallow layer: Adjust an oven rack to the middle position and preheat to 175°C. Line a 9-inch square baking pan with enough foil to create a 2-inch overhang on two sides. Set aside.
2. Melt the butter in a large saucepan over medium heat until light brown specks start to form and the butter starts to smell nutty, swirling the pan occasionally, about 10 minutes. Pour immediately into a large mixing bowl, being sure to scrape the brown bits from the bottom, and cool slightly, about 5 minutes. Add the brown sugar and 1 teaspoon salt and stir with a spatula or wooden spoon until smooth. Stir in the egg until incorporated.
3. Stir in the flour and graham cracker crumbs until just combined. Use your hands to mix the dough until well combined. Pat the dough evenly into the bottom of the prepared pan. Spread the marshmallow creme on top, using a small offset spatula to smooth it to the edge of the pan. Set aside.
4. For the chocolate brownie layer: Place the butter and 4 ounces of the chocolate into a heat-safe bowl. Heat in the microwave in 30 second increments until the chocolate is mostly melted, about 1 minute total. Stir until the chocolate is completely melted and smooth. Cool slightly. Stir in the sugar and 1/2 teaspoon salt until well combined. Stir in the eggs. Fold in the flour until just combined. Pour the brownie batter on top of the marshmallow, using a small offset spatula to smooth the batter to the edge of the pan. Sprinkle the remaining chocolate chunks over the top of the batter.
5. Bake until the marshmallow is lightly golden brown and a cake tester inserted into the center of the brownie comes out clean (it's okay if marshmallow sticks to the cake tester but there should be no runny chocolate brownie batter), about 30 minutes. (While the brownie is baking, the marshmallow will slowly start to rise to the top of the pan, causing a marshmallow swirl throughout the brownie.)
6. Cool completely in the pan. Once the brownies are cool, gently separate from the side of the pan using a small offset spatula or paring knife. Use the foil to carefully lift the brownies from the pan onto a cutting board. Discard the foil and cut the brownies into 16 squares.
Cook's Note: Marshmallow creme is super sticky. Spray your measuring cup and any tools that you use with nonstick cooking spray before working with it.
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