Preheat the oven to 350°F (180°C). Line three baking sheets with parchment paper.
In a medium-sized bowl or the bowl of a stand mixer fitted with the paddle attachment, whisk together the flour, salt, baking soda and cornstarch.
In a separate large bowl, add the butter, brown sugar and granulated sugar. Beat until light and fluffy, scraping the sides as needed. Add the milk and vanilla and beat again until creamy.
Add about half of the flour mixture to the butter mixture and mix until combined. Add the rest of the flour mixture and mix until just combined. Fold in the chocolate chips, graham cracker pieces, and mini marshmallows.
Using a cookie scooper, create 1 1/2 tablespoon (28 g) balls of dough and place them on the prepared baking sheets, leaving about 2 inches (5 cm) between the balls of dough for spreading.
Bake for 9-11 minutes, or until they are slightly golden-brown - they may look slightly underbaked, but will firm up as they set