Preheat your oven to 350°F and line a 9x13-inch baking pan with parchment paper for easy removal. This ensures your cozy s'mores cookies bars recipe releases perfectly without crumbling or sticking to edges.

Whisk together flour, baking soda, and salt in a small bowl, setting aside. Combine the dry ingredients thoroughly to distribute leavening evenly throughout your dough.

Cream together softened butter, granulated sugar, and brown sugar until light and fluffy—approximately 3-4 minutes. Beat in eggs one at a time, then add vanilla extract, mixing until smooth and fully incorporated.

Gradually fold the dry ingredients into the wet mixture, stirring until just combined. Add graham cracker crumbs to this base, distributing them evenly throughout the dough.

Press the dough firmly into your prepared pan, creating an even layer across the bottom. Sprinkle chocolate chips across the surface, then scatter milk chocolate chunks for additional richness.

Layer marshmallows across the top in an even distribution, pressing them down slightly so they nestle into the chocolate. Bake for 22-25 minutes until the edges turn golden brown and marshmallows begin toasting slightly.

Remove from the oven and allow cooling for 10 minutes in the pan, then transfer to a wire rack for complete cooling. This timing prevents the smores bars from falling apart while ensuring proper structure.