Preheat Oven: Start by heating your oven to 350 degrees F. Prepare your baking sheets by lining them with parchment paper or a Silpat.

Mix Dry Ingredients: In a bowl, combine the flour, cornstarch, baking soda, and salt. Whisk them together for about 20 seconds to ensure they are well mixed.

Cream Butter and Sugars: In the bowl of an electric mixer fitted with the paddle attachment, cream together the softened butter, brown sugar, and granulated sugar until the mixture is light and fluffy, which should take about 1 minute.

Add Eggs and Vanilla: Beat in the eggs one at a time, then mix in the vanilla extract. Make sure each egg is fully incorporated before adding the next.

Combine with Dry Ingredients: With the mixer set on low speed, gradually add the dry ingredients to the wet mixture, blending until just combined. Avoid overmixing to ensure soft cookies.

Add Chocolate Chips: Stir in one bag (12 oz) of the chocolate chips into the batter. Reserve the second bag for later use.

Form Cookies: Scoop the cookie dough into 2-tablespoon sized balls and place them on the prepared baking sheets, spacing them about 2 inches apart.

Bake: Bake in the preheated oven for 9-12 minutes, or until the edges are just turning golden. Allow the cookies to cool completely on the baking sheets.

Prepare for Assembly: Melt the second bag of chocolate chips in a microwave-safe bowl in 30-second increments at 50% power, stirring between each increment until smooth. Spread a layer of melted chocolate on the flat side of half the cookies.

Toast Marshmallows: Over a fire pit or stove burner, toast the marshmallows until they are golden brown. Be careful if using a stove to keep marshmallows away from direct heat.

Assemble: Place two toasted marshmallows on top of the chocolate-covered cookies and then top with another cookie to form a sandwich. Drizzle with extra melted chocolate for added indulgence.

Serve Immediately: Enjoy these decadent s'mores while the marshmallows are warm and gooey for the best experience.