Preheat your oven to 350°. Grease a 9x13 baking pan.
Place graham cracker sheets across the bottom of pan, fill up every spot and break grahams in half if you have to. Set aside.
Make Dough
1. Melt your butter halfway in microwave. Cream butter and brown sugar together until smooth.
2. Add in the egg, egg yolk and vanilla and mix until completely combined.
3. Incorporate the flour, corn starch and baking soda on low until JUST combined.
4. Pour in your chocolate chunks and marshmallows and mix them in by hand with a rubber spatula or spoon until they are evenly distributed.
5. Take big chunks of dough and press down on top of graham crackers (it's much easier to just use your hands for this part) until no grahams are seen and everything is even and level.
6. Place in your preheated oven and bake for 17 minutes. While this is baking, start getting your marshmallows ready and chopping up your chocolate bar. You want big pieces.
7. Once pan is out of the oven, place marshmallows and chocolate pieces all over the top of the bars where you want them. Place back in the oven to bake another 5 minutes.
8. Once 5 minutes is up, take the bars out of the oven and let them cool for about 20 minutes or until the chocolate is set enough to be able to cut. (Waiting is the hardest part!)
9. Once cooled, slice them up (I like to use a pizza cutter for easy cutting), serve and enjoy!!