Preheat oven to 350°F. Grease a 9x13-inch pan, line with parchment paper, and set aside.
Whisk graham cracker crumbs, flour, baking powder, and salt in a bowl.
In another bowl, beat softened butter, white sugar, and brown sugar for 2 minutes. Mix in eggs and vanilla until smooth.
Add dry ingredients to the butter mixture and mix until combined. Divide dough into two balls.
Press one ball evenly into the pan. Layer chocolate bars on top, then spread marshmallow fluff over the chocolate.
Roll or flatten the second dough ball to fit the pan. Lay it on top of the marshmallow layer, covering as much as possible.
Bake for 30–35 minutes until golden brown. Cool completely on a rack before slicing and serving.