Preheat oven to 350°F (175°C).

In a large mixing bowl, beat together the butter, brown sugar, and granulated sugar. Add the eggs and vanilla, and continue beating until light and fluffy.

In a separate bowl, combine the flour, salt, and graham cracker crumbs. Gradually mix the dry ingredients into the wet mixture until fully combined.

Divide the dough in half. Press one half into a greased 9×13-inch baking dish.

Lay three full chocolate bars side by side on top of the dough. Break the fourth bar into strips and fill in the spaces around the edges.

Spread the marshmallow fluff evenly over the chocolate layer.

Crumble the remaining dough into small pieces over the marshmallow layer and gently pat down.

Bake uncovered for 30–35 minutes, or until golden brown and puffed.

Cool slightly before slicing. Serve warm or at room temperature.