Preheat oven to 350 degrees F. Grease an 9x13-inch baking pan with non-stick spray. Set aside.
In a large bowl, beat together butter and brown sugar until combined and fluffy. Beat in the eggs and vanilla.
In a medium bowl, mix together the flour, graham cracker crumbs, baking powder and salt. Gradually add this mixture to the butter mixture. On low speed, mix until just combined.
Divide dough in half and press one half of dough into the bottom of the pan evenly. Place the 6 chocolate bars over the top of dough, in a single layer. (You may have to break some of them into pieces so they fit into the pan.) Spread the marshmallow creme/ fluff over the chocolate. (You can run a spoon under hot water to help spread it easier.)
Place the remaining dough over the marshmallow creme/fluff. (The easiest way to do this is to take pieces of the dough and flatten them in your hand, then lay over the marshmallow. It's okay if it doesn't cover it all completely.)
Bake for 28 to 30 minutes or until golden-brown. Remove from oven. Cool completely before cutting into bars.