Preheat your oven to 350°F (175°C) and line an 8x8 inch baking pan with parchment paper.

Cream together butter, brown sugar, and granulated sugar until smooth and fluffy, about 2-3 minutes.

Add the egg and vanilla extract, beating well until fully combined.

In a separate bowl, whisk together flour, graham crackers, baking soda, and salt. Gradually add to the wet ingredients and mix until just combined.

Press half of the dough evenly into the bottom of the lined baking pan.

Layer pieces of Hershey's chocolate bars over the pressed dough, then spread marshmallow fluff on top.

Drop spoonfuls of the remaining dough over the marshmallow fluff, gently spreading it to cover.

Bake for 25 minutes until edges are set and golden brown, then let cool completely on a wire rack before slicing.