Preheat oven to 325°F. Line a baking sheet with parchment paper. Set aside.
To a large mixing bowl, add butter, shortening, sugars, eggs, and vanilla. Using a hand mixer, beat ingredients together on medium speed until fully combined. Set aside.
In a separate bowl, whisk together the flour, cornstarch, salt, and baking powder.
Add the flour mixture to the butter mixture. Mix on medium speed until just combined.
Fold in the chocolate chunks and chocolate chips.
Using a ¼ cup, set the dough onto a lined cookie sheet, about 2 inches apart. Using your hands or the bottom of a glass, shape and slightly flatten each ball of dough into a disc shape.
Top each cookie with three pieces of the milk chocolate bar, arranging them in a triangular shape. Then, add one heaping teaspoon of marshmallow fluff to the center of each cookie, creating a small well to keep the marshmallow from spilling over while the cookies are baking.
Sprinkle approximately ½ teaspoon of graham cracker crumbs on top of the marshmallow fluff.
Bake for 12-13 minutes, or until the marshmallow fluff is slightly melted and the edges of the cookie are set.
Let the cookies cool for about 5-10 minutes before enjoying.