Grease a 9" x 9" baking dish with cooking spray, then line with parchment. In a medium bowl, mix flour, crushed crackers, salt, and baking soda.

In a large bowl, using a handheld mixer on medium-high speed, beat butter, brown sugar, and granulated sugar until smooth and creamy. Add egg and vanilla and beat until combined. Add dry ingredients and beat until a few dry streaks remain. Add chocolate, chopped crackers, and marshmallows and mix until just combined.

Press about two-thirds of cookie dough into prepared dish. Spread Marshmallow Fluff over top.

Scoop remaining cookie dough into about 2 Tbsp.-size pieces and flatten in your hands, then arrange over Fluff; it’s okay if the top isn’t completely covered. Transfer dish to freezer and freeze dough at least 45 minutes and up to overnight.

Arrange a rack in center of oven; preheat to 350°. Bake bar from frozen until edges feel set and center feels slightly soft, and golden brown, 45 to 50 minutes. Let cool slightly.

Using parchment overhang, lift out bar and transfer to a cutting board. Cut into squares.