Preheat oven to 350F (175C) and line a baking sheet with parchment paper. Set aside.

Combine butter and sugars in a large bowl (or the bowl of a stand mixer with paddle attachment) and use an electric mixer to beat on medium speed until light and creamy.

Add eggs and vanilla extract and stir into batter until completely combined.

In a separate bowl, whisk together flour, graham cracker crumbs, cornstarch, baking powder, baking soda, and salt.

With mixer on medium-low speed, gradually add dry ingredients to butter mixture just until completely combined.

Add 1 cup (40g) mini marshmallows, chocolate chips, and chocolate bars and use a spatula to stir until ingredients are well incorporated.

Scoop dough by heaping 1 ½ Tablespoon-sized ball and drop on prepared baking sheet, spacing at least 2” apart. For more uniform/round cookies, briefly roll dough between your palms to make a smooth ball before placing on cookie sheet. Any exposed marshmallows will melt onto the tray, this is fine (and why we line the tray with parchment paper) but to help keep the marshmallows in the cookie, try to tuck any exposed pieces of marshmallow into the dough.

Transfer cookies to center rack of 350F (175C) oven and bake for 10-12 minutes. While cookies are baking, use a knife to cut the remaining 1 cup (40g) marshmallows in half crosswise.

Within 1 minute of the cookies being removed from the oven, gently press marshmallow halves into the surface of the cookies.

Allow cookies to cool completely on baking sheet before enjoying.