Cream the softened butter, light brown sugar, and granulated sugar together for 3 minutes until light and fluffy.
Mix in the egg, egg yolk, and vanilla extract until just combined.
Mix in the flour, cocoa powder, chocolate chips, salt, baking soda, and baking powder until just combined.
Cover the bowl and chill the dough in the fridge for 1 hour.
Scoop the dough with a standard cookie scoop (two-tablespoons) onto a parchment-lined baking tray.
Then flatten each dough ball between your palms. Place one large marshmallow long side down in the center of the dough. Wrap and fold the dough up around the marshmallow, leaving just the top peaking through the dough. See post photos for visual example.
Place them back on the baking tray. Top with crushed graham cracker pieces and crumbs.
Chill the tray of cookie dough for another 1-2 hours in the fridge. Then preheat the oven to 350 F/180 C.
Bake the cookies spaced 3 inches apart on a parchment-lined baking tray for 10-11 minutes until the marshmallow is puffy and the dough is starting to split open around it.
Right out of the oven, use a large round cookie cutter to scoot the edges in and make them perfectly round. Stick a square of chocolate on top.
Let the tray cool on a wire cooling rack. Then carefully remove them from the pan. The marshmallow may stick to the parchment paper, so you may need to use a spatula to gently remove them from the tray. Enjoy!