Heat oven to 350 degrees. Prepare a 9-inch square baking pan by spraying with cooking spray or lining it with parchment paper. (Using parchment paper with the sides over hanging, helps lift the bars out of the pan easily.) In a large bowl, using a handheld or standing mixer cream butter and sugar until light and fluffy, about 1-2 minutes. Beat in egg and vanilla. Add the flour, cracker crumbs, baking powder and salt. Beat together on low speed just until combined. The dough will be crumbly. Set aside 1 cup of cookie dough for topping. Press the remaining into the prepared 9-inch square baking pan and press down to form a crust. Bake at 350 degrees for 15 minutes or until golden brown. Let cool completely, and then arrange milk chocolate bars over the graham cracker crust. You may have to break one or two into the square pieces to ensure that everything gets covered. Drop spoonful’s of the marshmallow cream over the chocolate and then spread out. Sprinkle the reserved cookie dough on top and gently press down. Sprinkle mini marshmallows and chocolate bits over top. Cook for 23-25 minutes. The edges will be golden brown. Let cool in the pan for at least 30 minutes or longer. Serve room temperature or chilled.