Preheat oven to 350°F and line a baking sheet with parchment paper.
Roughly chop the Hershey’s chocolate bar and graham crackers. Tear half of the mini marshmallows in half.
In a medium bowl, whisk together the flour, baking powder, baking soda, cornstarch, and salt.
In a large mixing bowl, beat the butter, brown sugar, and granulated sugar together until light and creamy, about 2–3 minutes.
Add the egg, egg yolk, and vanilla extract. Mix until fully combined.
Gradually add the dry ingredients into the wet ingredients and mix until just combined.
Fold in the chocolate chips, chopped Hershey’s chocolate, crushed graham crackers, and most of the marshmallows. Reserve some marshmallows and chocolate for topping.
Cover and chill the dough for at least 1 hour.
Scoop large 2–3 tablespoon sized balls of dough onto the prepared baking sheet, spacing them about 2 inches apart.
Bake for 8 minutes. Remove from the oven and gently press extra marshmallows and chocolate pieces onto the tops of the cookies.
Return to the oven and bake for another 3–5 minutes until the edges are golden brown and the marshmallows are toasted.
Sprinkle with flaky sea salt if desired and allow the cookies to cool on the baking sheet for 10 minutes before serving.