Preheat the oven to 350ºF. Butter and line a glass 9x9 pan with parchment, and then butter the paper.
Melt half a stick of butter, and mix it in with the graham cracker crumbs. Press the moistened crumbs into the bottom of your prepared pan.
In the bowl of the stand mixer, cream together the remaining stick of softened butter, sugar, and light brown sugar.
Add in the eggs and vanilla, and beat to combine.
Sift in the flour, salt, and baking soda, and fully combine.
Transfer half the batter to a small mixing bowl and stir in the cocoa powder. Add ¼ cup of chocolate chips to both bowls of batter.
Alternately dollop the plain and chocolate batters over the graham cracker crust. Gently smooth the batter into the corners and edges without mixing it up. Then, use the spatula to give it a pretty swirl pattern, again, careful not to mix the batters together too much.
Sprinkle with additional chocolate chips and bake for 25 minutes until a toothpick comes out clean.
Top your marble brookie bars with halved marshmallows, and place under the broiler, set on LOW, for 3 minutes until the marshmallows are golden.
All the bars to cool completely before cutting into 9 even bars.
Notes
*Optional: Melt 2 Tablespoons of chocolate with a dab of butter to drizzle over the top of your bars, and sprinkle on more crushed graham cracker.
Keyword Brownies, Chocolate, Chocolate Chip, Cookies, Graham Cracker Crust, Marshmallows