Preheat your oven to 350°F (175°C) and grease a 9x13 inch baking pan.
In a medium bowl, whisk together flour, baking soda, salt, and graham cracker crumbs.
In a large bowl, beat the egg with brown sugar, melted butter, and vanilla extract until smooth.
Gradually add the dry ingredients to the wet ingredients and mix until just combined.
Fold in the chocolate chips and mini marshmallows gently.
Spread the batter evenly into the prepared baking pan and bake for 25 minutes or until edges are golden and a toothpick inserted near the center comes out clean.
Let the bars cool completely in the pan before cutting into squares.
Notes
For extra gooeyness, add more marshmallows on top halfway through baking.