Preheat the oven to 350°F (177°C).

Line a 9x9-inch baking pan with parchment paper, allowing some overhang on the sides for easy removal.

In a medium bowl, whisk together the flour, crushed graham crackers (bigger pieces are just fine. they add texture), baking soda, and salt. Set aside.

In a large mixing bowl, cream together the softened butter, brown sugar, and granulated sugar until light and fluffy, about 2–3 minutes.

Add the eggs one at a time, mixing well after each addition. Mix in the vanilla extract and sourdough discard until fully incorporated.

Gradually add the dry ingredients and mix just until combined.

Fold in the chopped Hershey's chocolate bars (2 bars) and the chocolate chips.

Press approximately two-thirds of the cookie dough evenly into the bottom of the prepared pan.

Arrange the marshmallows in a single layer over the dough, using about 22–25 marshmallows to cover most of the surface.

Using the remaining one-third of the cookie dough, scoop small portions over the marshmallows. Gently press the dough down slightly, leaving some marshmallows partially visible. The marshmallows do not need to be completely covered.

Bake for 25–32 minutes, or until the bars are golden brown and the cookie dough is fully baked through.

About 5 minutes before the bars are finished baking, remove the pan from the oven and sprinkle the remaining crumbled Hershey's chocolate bar evenly over the top.

Return the pan to the oven and bake for the remaining 5 minutes, allowing the chocolate to melt into the bars.

Allow the bars to cool completely in the pan before lifting them out using the parchment paper overhang.

Slice into 16 squares and serve.