Set your oven to 350°F. Line a 9-inch square baking dish with foil, ensuring it is greased generously.

In a bowl, combine graham cracker crumbs with sugar, then blend in the melted butter until thoroughly mixed. Press this mixture firmly into the prepared pan and bake for approximately 10 minutes.

For the brownies, combine water, unsalted butter, sugar, and cocoa powder in a pot over medium heat. Cook for 2-3 minutes until completely blended. Do not worry about thickness. Stir in the vegetable oil and vanilla extract, cooking for an additional minute.

Remove from heat and let sit for 5 minutes. Then add the egg and egg yolk, whisking until everything is well combined. Add in the baking soda, salt, and cornstarch.

Carefully fold in the flour with a spatula until just combined. Just before fully incorporating the flour, add the chocolate chunks to the mix. Avoid overmixing!

Pour the prepared brownie batter into the pan and bake for 25-27 minutes.

After baking, sprinkle chocolate pieces and mini marshmallows evenly on top. Set the broiler to low and place the brownies under the heat for a few minutes, keeping an eye to prevent burning!

Allow the brownies to cool completely before slicing. For clean cuts, rinse your knife in hot water and wipe it clean after each slice. Enjoy!