Preheat oven to 350°F. Grease a 9-inch square baking pan and set aside.
To make the topping, in a large bowl, whisk together the flour, graham cracker crumbs, and brown sugar. Add the butter and cut in with a pastry blender or your hands until coarse crumbs form. Stir in the chocolate chips and marshmallow bits and set aside.
To make the cake batter, whisk together 1 cup of flour, graham cracker crumbs, baking powder, baking soda and salt. Set aside.
In a separate bowl, cream together the butter and sugar until light and fluffy, about 3 minutes. Stir in the eggs, one at a time, and then add the vanilla extract. Mix together the buttermilk and vanilla.
Stir in the dry ingredients alternating with the buttermilk mixture, starting and ending with dry ingredients and mixing until just combined after each addition. Toss the chocolate chips with the remaining 1 tablespoon flour and then fold into the batter until just combined.
Pour half of the batter into the prepared pan and spread out into an even layer. Top with half of the crumb mixture and then add the remaining batter and spread out once more. Top with the rest of the topping. Bake until golden brown and a toothpick, inserted in the middle, comes out clean, about 30 to 35 minutes. Remove from the oven and allow to cool completely before slicing and serving. Enjoy!