01 - Preheat the oven to 350°F (175°C) and line two light-colored baking sheets with parchment paper.
02 - In the bowl of a stand mixer, cream the softened butter, packed light brown sugar, and granulated sugar until light and fluffy.
03 - Beat in the vanilla extract and the egg, scraping down the sides of the bowl as needed.
04 - In a separate medium bowl, whisk together the flour, baking powder, baking soda, and salt. Add the dry ingredients to the mixer and blend just until combined.
05 - Stir in the semi-sweet chocolate chips by hand until evenly distributed.
06 - Arrange six graham cracker halves on each baking sheet. Place pieces of milk chocolate bar and one marshmallow on each graham cracker half.
07 - Use a large cookie scoop to portion about 3 tablespoons of dough per cookie. Slightly flatten each dough ball into a disc and place on top of the marshmallow stacks. Top each with additional chocolate chips if desired.
08 - Bake for 10 to 13 minutes, or until the cookie edges are light golden brown and the marshmallows have melted.
09 - Allow cookies to cool on the baking sheets before serving.