Preheat oven to 350° F. In a standing mixer, mix together white sugar, brown sugar, and butter (½ inch cubes) starting on low speed and gradually increasing to high. Mix on high for about 4-5 minutes or until creamy.

In a separate bowl, whisk together flour, graham cracker crumbs, salt, cornstarch, and baking soda. Set aside.

Add vanilla and eggs to the sugar/butter mixture and mix on medium-high until well combined. Scrape down sides of the bowl to make sure it is evenly mixed. Add the dry ingredients to the standing mixer and mix on lowest speed until just barely combined. Don't overmix.

Using a rubber spatula, stir in the marshmallows and chocolate chips.

Either use parchment paper (preferred) to line a 13x9 glass pan, or spray it with cooking spray. Press dough into the pan. Bake for 15-20 minutes. These don't need to be completely cooked because they will be put back in the oven for at least another 5 minutes. The bars should be set, and just barely starting to brown.

Evenly distribute topping over the top of the bars. You don't need to use the exact quantities for the topping, add more or less according to your preference!

Bake for about another 5-7 minutes, until marshmallows start to brown. Let cool for at least 10 minutes and enjoy!