Heat your oven to 350 degrees F. Line a 9x13 inch baking pan with parchment paper, leaving a couple inches of overhang on the long sides so you can lift the whole slab out later. Spray the parchment lightly with cooking spray.

Grate the zucchini on the large holes of a box grater. Wrap it in a clean kitchen towel and squeeze hard over the sink to remove as much liquid as possible. Measure 1 1/4 cups of the squeezed zucchini and set aside.

Whisk together the flour, baking soda, and salt in a bowl and set aside. In a large bowl, beat the softened butter with both sugars until creamy and smooth, about 2 minutes of vigorous stirring. Add the eggs and vanilla and mix until combined.

Stir the flour mixture into the butter mixture until just combined, then fold in the squeezed zucchini and half of the chocolate chips (1/2 cup). The dough will be thick and a little sticky.

Scoop the dough into the prepared pan and press it into an even layer using lightly dampened fingertips or the back of a spoon. Get it as level as you can all the way to the edges.

Scatter the mini marshmallows evenly over the top of the dough, then sprinkle on the remaining 1/2 cup of chocolate chips, followed by the crushed graham crackers. Press the toppings very gently into the surface so they stick.

Bake on the center rack for 20 to 22 minutes, until the edges are golden brown, the marshmallows are toasted and puffed, and the center feels set when you gently press it. The bars should not jiggle in the center.

Let the pan cool completely on a wire rack before lifting the slab out using the parchment overhang. This takes at least 1 hour. Do not rush this step or the bars will fall apart when cut. Once fully cooled, slice into 24 bars with a sharp knife.