Place the chilled cookie dough in the muffin tin into the preheated oven.
Bake for approximately 15 minutes or until edges are pale golden and the middles no longer look wet. Remove from the oven. Working quickly, use a shot glass or a small teaspoon to press down into the centre and sides of each cookie, creating a well or cup with a crust about ¼-inch thick around the edge. Place cookies back into the oven to dry out the sides and bottoms of the cups; approximately 5–7 minutes.
Remove from oven and sprinkle a heaping teaspoon of the graham cracker crumble into the centre of each cookie.
With kitchen scissors or a sharp knife, cut 6 large marshmallows in half (lengthwise). Press a halved marshmallow, cut side down into each well, over the crumble. Place back into the oven to gently melt the marshmallows; 5 minutes.
Remove tin from oven. With oven mitts, move the oven rack to the top third of the oven. Preheat oven to broil.
Once preheated, place tin back into the oven to toast marshmallows; approximately 1–3 minutes. Watch carefully to ensure marshmallows do not burn. Rotate pan as necessary. Remove from oven. If a blowtorch is available, use the blowtorch to toast each marshmallow instead of broiling.
With a small spoon, gently indent each toasted marshmallow to allow room for the milk chocolate ganache.
Divide the chocolate ganache amongst the cookie cups (approximately 2 teaspoons in each cup), covering the toasted marshmallow. Sprinkle the entire surface of the ganache with graham cracker crumble.
Place into the fridge to set slightly; 15 minutes.
Cut another 6 large marshmallows in half lengthwise. Place a halved marshmallow, cut-side down, on top of the graham cracker crumble. Return to broiler or use a blow torch to toast the marshmallows until desired darkness is achieved.
Store cookies in an airtight container at room temperature for up to 3 days or in the fridge for up to a week.