• Preheat the oven to 350F then line a baking sheet with parchement paper and set aside.
• Cream butter, brown sugar, and granulated sugar until light and creamy. Mix in the egg and the vanilla butter emulsion until combined.
• Add the flour, baking soda, and salt and mix until the flour mixture disappears.
• Divide the cookie dough into two bowls, remove two tablespoons of cookie dough from one, and put it into the other.
• Add the cocoa powder to the bowl with the extra 2 tablespoon of cookie dough, and mix until combined. Add the graham crackers, and chocolate chips to the other bowl and mix until combined.
• Using a 2 tablespoon scoop, scoop out the chocolate cookie dough. Place it on the baking sheet first, and flatten it about ½ inch in thickness.
• Scoop2 tablespoon worth of the graham cracker cookie dough and place it on top of the flattened chocolate cookies, press just a little to get the top flat enough to add in the brownie batter (don't press down too much). Top each cookie with ½ tablespoon of brownie batter (see make brownie batter step), and sprinkle some graham cracker crumbs on top.
• Bake in the preheated oven for 9 minutes. Place half a large marshmallow on top and bake for 3 more minutes. Once the cookies are baked, let them cool down on the baking sheet for 10 minutes before transferring them to a cooling rack. This is an optional step, but you could also toast the marshmallow for extra pizaz.

Make brownie batter

• In a microwave-safe bowl, melt the butter and the chocolate chips. Add in the egg yolk and brown sugar and whisk until combined. Mix in the flour.