Preheat oven to 350° Fahrenheit. Line baking dish with parchment paper, leaving excess parchment paper overhanging two sides of dish. Set dish aside.
Add ½ cup unsalted butter and ¾ cup light brown sugar to mixing bowl. Use hand mixer or stand mixer on medium speed to combine ingredients into pale, fluffy mixture, approximately 2 to 3 minutes.
Once butter and sugar are creamed, add 1 large egg and 1 teaspoon pure vanilla extract to bowl. Mix until ingredients are fully incorporated and mixture is smooth. Set aside.
Add ½ teaspoon baking powder, 1 cup graham cracker crumbs, and ¼ teaspoon salt to medium mixing bowl. Whisk ingredients together until thoroughly blended.
Slowly add graham cracker mixture into butter mixture, mixing ingredients together until just combined into thick dough.
Once dough has formed, transfer approximately half to two-thirds of dough to parchment-lined baking dish. Press dough out into flat, even layer, covering bottom of baking dish completely.
Pour 1 7-ounce jar marshmallow creme on top of dough layer. Spread marshmallow creme out evenly across dough, wetting spatula with hot water as needed to make spreading marshmallow creme easier.
Top layer of marshmallow creme with pieces of 2 large chocolate bars. Chocolate does not need to cover marshmallow creme completely.
Divide remaining graham cracker dough into smaller portions. Flatten each portion of dough evenly, then lay portions of dough on top of chocolate layer. Repeat until all dough has been added to baking dish. Note: top layer of dough will not cover chocolate completely.
Place baking dish in preheated oven. Bake 30 minutes or until toothpick inserted into center of smores bars is not covered in graham cracker crumbs when removed.
Remove baking dish from oven and place on wire cooling rack. Allow bars to cool completely.