Preheat oven to 325°F. Spray a 8X8 inch pan with baking non-stick spray. Line bottom and 2 sides of the pan with parchment paper. Spray again.
Use a food processor to turn graham cracker to fine graham cracker crumbs. Mix graham cracker crumbs, brown sugar, and melted butter together with a fork. Pour into the pan. Use the back of a measuring cup to compact the crust. The crust should be on the bottom and not up the sides.
Bake for 8 minutes. Allow to cool.
Sift the flour, cocoa powder, cornstarch, and salt. In a mixing bowl with a whisk attachment, beat the eggs, egg yolk, and sugar for 5 minutes on high. While waiting, heat butter in the microwave and allow it to cool slightly.
Add in melted butter, oil, and vanilla. Mix on low until combined. Take off mixer. Pour in dry ingredients. Gently fold with a rubber spatula until combined. Fold in chocolate chips.
Pour batter over graham cracker crust. Bake for 40-50 minutes. The edges will be set and the middle will be slightly underdone. Sprinkle mini marshmallows on top and allow to cool for 20 minutes.
Scoop marshmallow creme into a microwave safe bowl. Heat for 20-30 seconds until melty. Pour over mini marshmallows. Spread evenly.
Pour chocolate chips into a bowl. Heat heavy cream in a sauce pan over medium-low heat. Heat until hot and steaming. Pour over chocolate chips. Let it sit for 1 minute and 30 seconds. Mix until smooth.
Place in fridge for 45 minutes. Mix every 10-15 minutes. Spread on top of cooled brownies.
Place brownies in the freezer for 15 minutes before cutting. Top with sea salt right before serving.
High altitude baking- Add an ½ tablespoon of flour to the brownies.