In a large mixing bowl, combine flour, baking powder, baking soda, and salt. Chop the dark chocolate into chunks and add them to the flour mixture. Crush the Graham crackers by hand and mix them into the dry mixture.
In a separate bowl or measuring cup, whisk together the sugars, oil, and milk until the mixture is smooth and incorporated, approximately 2 minutes. Ensure the brown sugar is free of lumps. Break any lumps using your hands or a spoon before adding.
Add the sugar mixture to the flour mixture, then stir with a wooden spoon or spatula until just combined and no dry flour is visible. Avoid overmixing. For hot days, briefly place the dough in the freezer to keep it cold.
If you're using regular-sized marshmallows, cut them in half using scissors.
To form cookie balls, scoop the dough with an ice cream scoop and create a cavity in the middle using your thumb. Place half a marshmallow inside and cover it with more cookie dough. Release the ball from the scoop and roll it to ensure the marshmallow is fully covered. Repeat until all the dough is used. Refrigerate the cookie dough balls overnight (for at least 12 hours). This step is important; do not skip it.
Preheat the oven to 180°C / 355°F. Line two cookie sheets with parchment paper. Keep the cookies in the fridge until just before baking.
Sprinkle the dough balls with coarse-grained sea salt and bake for 10 to 13 minutes, or until the edges are golden.
If desired, top the cookies with additional graham cracker crumbs, chocolate, and mini marshmallows while they are still hot and soft. Enjoy the cookies while they're warm!